var_dump

Keep in mind if you have xdebug installed it will limit the var_dump() output of array elements and object properties to 3 levels deep.
To change the default, edit your xdebug.ini file and add the folllowing line:
xdebug.var_display_max_depth=n
More information here:
http://www.xdebug.org/docs/display 

If you're like me and uses var_dump whenever you're debugging, you might find these two "wrapper" functions helpful.
This one automatically adds the PRE tags around the var_dump output so you get nice formatted arrays.
<?php
function var_dump_pre($mixed = null) {
 echo '<pre>';
 var_dump($mixed);
 echo '</pre>';
 return null;
}
?>

This one returns the value of var_dump instead of outputting it.
<?php
function var_dump_ret($mixed = null) {
 ob_start();
 var_dump($mixed);
 $content = ob_get_contents();
 ob_end_clean();
 return $content;
}
?>

Fairly simple functions, but they're infinitely helpful (I use var_dump_pre() almost exclusively now).

I post a new var_dump function with colors and collapse features. It can also adapt to terminal output if you execute it from there. No need to wrap it in a pre tag to get it to work in browsers. 
<?php
function dump_debug($input, $collapse=false) {
 $recursive = function($data, $level=0) use (&$recursive, $collapse) {
 global $argv;
 $isTerminal = isset($argv);
 if (!$isTerminal && $level == 0 && !defined("DUMP_DEBUG_SCRIPT")) {
 define("DUMP_DEBUG_SCRIPT", true);
 echo '<script language="Javascript">function toggleDisplay(id) {';
 echo 'var state = document.getElementById("container"+id).style.display;';
 echo 'document.getElementById("container"+id).style.display = state == "inline" ? "none" : "inline";';
 echo 'document.getElementById("plus"+id).style.display = state == "inline" ? "inline" : "none";';
 echo '}</script>'."\n";
 }
 $type = !is_string($data) && is_callable($data) ? "Callable" : ucfirst(gettype($data));
 $type_data = null;
 $type_color = null;
 $type_length = null;
 switch ($type) {
 case "String": 
 $type_color = "green";
 $type_length = strlen($data);
 $type_data = "\"" . htmlentities($data) . "\""; break;
 case "Double": 
 case "Float": 
 $type = "Float";
 $type_color = "#0099c5";
 $type_length = strlen($data);
 $type_data = htmlentities($data); break;
 case "Integer": 
 $type_color = "red";
 $type_length = strlen($data);
 $type_data = htmlentities($data); break;
 case "Boolean": 
 $type_color = "#92008d";
 $type_length = strlen($data);
 $type_data = $data ? "TRUE" : "FALSE"; break;
 case "NULL": 
 $type_length = 0; break;
 case "Array": 
 $type_length = count($data);
 }
 if (in_array($type, array("Object", "Array"))) {
 $notEmpty = false;
 foreach($data as $key => $value) {
 if (!$notEmpty) {
 $notEmpty = true;
 if ($isTerminal) {
 echo $type . ($type_length !== null ? "(" . $type_length . ")" : "")."\n";
 } else {
 $id = substr(md5(rand().":".$key.":".$level), 0, 8);
 echo "<a href=\"javascript:toggleDisplay('". $id ."');\" style=\"text-decoration:none\">";
 echo "<span style='color:#666666'>" . $type . ($type_length !== null ? "(" . $type_length . ")" : "") . "</span>";
 echo "</a>";
 echo "<span id=\"plus". $id ."\" style=\"display: " . ($collapse ? "inline" : "none") . ";\">&nbsp;&#10549;</span>";
 echo "<div id=\"container". $id ."\" style=\"display: " . ($collapse ? "" : "inline") . ";\">";
 echo "<br />";
 }
 for ($i=0; $i <= $level; $i++) {
 echo $isTerminal ? "| " : "<span style='color:black'>|</span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;";
 }
 echo $isTerminal ? "\n" : "<br />";
 }
 for ($i=0; $i <= $level; $i++) {
 echo $isTerminal ? "| " : "<span style='color:black'>|</span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;";
 }
 echo $isTerminal ? "[" . $key . "] => " : "<span style='color:black'>[" . $key . "]&nbsp;=>&nbsp;</span>";
 call_user_func($recursive, $value, $level+1);
 }
 if ($notEmpty) {
 for ($i=0; $i <= $level; $i++) {
 echo $isTerminal ? "| " : "<span style='color:black'>|</span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;";
 }
 if (!$isTerminal) {
 echo "</div>";
 }
 } else {
 echo $isTerminal ? 
 $type . ($type_length !== null ? "(" . $type_length . ")" : "") . " " : 
 "<span style='color:#666666'>" . $type . ($type_length !== null ? "(" . $type_length . ")" : "") . "</span>&nbsp;&nbsp;";
 }
 } else {
 echo $isTerminal ? 
 $type . ($type_length !== null ? "(" . $type_length . ")" : "") . " " : 
 "<span style='color:#666666'>" . $type . ($type_length !== null ? "(" . $type_length . ")" : "") . "</span>&nbsp;&nbsp;";
 if ($type_data != null) {
 echo $isTerminal ? $type_data : "<span style='color:" . $type_color . "'>" . $type_data . "</span>";
 }
 }
 echo $isTerminal ? "\n" : "<br />";
 };
 call_user_func($recursive, $input);
}
?>

<?php
/**
 * Better GI than print_r or var_dump -- but, unlike var_dump, you can only dump one variable. 
 * Added htmlentities on the var content before echo, so you see what is really there, and not the mark-up.
 * 
 * Also, now the output is encased within a div block that sets the background color, font style, and left-justifies it
 * so it is not at the mercy of ambient styles.
 *
 * Inspired from: PHP.net Contributions
 * Stolen from: [highstrike at gmail dot com]
 * Modified by: stlawson *AT* JoyfulEarthTech *DOT* com 
 *
 * @param mixed $var -- variable to dump
 * @param string $var_name -- name of variable (optional) -- displayed in printout making it easier to sort out what variable is what in a complex output
 * @param string $indent -- used by internal recursive call (no known external value)
 * @param unknown_type $reference -- used by internal recursive call (no known external value)
 */
function do_dump(&$var, $var_name = NULL, $indent = NULL, $reference = NULL)
{
 $do_dump_indent = "<span style='color:#666666;'>|</span> &nbsp;&nbsp; ";
 $reference = $reference.$var_name;
 $keyvar = 'the_do_dump_recursion_protection_scheme'; $keyname = 'referenced_object_name';
 
 // So this is always visible and always left justified and readable
 echo "<div style='text-align:left; background-color:white; font: 100% monospace; color:black;'>";
 if (is_array($var) && isset($var[$keyvar]))
 {
 $real_var = &$var[$keyvar];
 $real_name = &$var[$keyname];
 $type = ucfirst(gettype($real_var));
 echo "$indent$var_name <span style='color:#666666'>$type</span> = <span style='color:#e87800;'>&amp;$real_name</span><br>";
 }
 else
 {
 $var = array($keyvar => $var, $keyname => $reference);
 $avar = &$var[$keyvar];
 $type = ucfirst(gettype($avar));
 if($type == "String") $type_color = "<span style='color:green'>";
 elseif($type == "Integer") $type_color = "<span style='color:red'>";
 elseif($type == "Double"){ $type_color = "<span style='color:#0099c5'>"; $type = "Float"; }
 elseif($type == "Boolean") $type_color = "<span style='color:#92008d'>";
 elseif($type == "NULL") $type_color = "<span style='color:black'>";
 if(is_array($avar))
 {
 $count = count($avar);
 echo "$indent" . ($var_name ? "$var_name => ":"") . "<span style='color:#666666'>$type ($count)</span><br>$indent(<br>";
 $keys = array_keys($avar);
 foreach($keys as $name)
 {
 $value = &$avar[$name];
 do_dump($value, "['$name']", $indent.$do_dump_indent, $reference);
 }
 echo "$indent)<br>";
 }
 elseif(is_object($avar))
 {
 echo "$indent$var_name <span style='color:#666666'>$type</span><br>$indent(<br>";
 foreach($avar as $name=>$value) do_dump($value, "$name", $indent.$do_dump_indent, $reference);
 echo "$indent)<br>";
 }
 elseif(is_int($avar)) echo "$indent$var_name = <span style='color:#666666'>$type(".strlen($avar).")</span> $type_color".htmlentities($avar)."</span><br>";
 elseif(is_string($avar)) echo "$indent$var_name = <span style='color:#666666'>$type(".strlen($avar).")</span> $type_color\"".htmlentities($avar)."\"</span><br>";
 elseif(is_float($avar)) echo "$indent$var_name = <span style='color:#666666'>$type(".strlen($avar).")</span> $type_color".htmlentities($avar)."</span><br>";
 elseif(is_bool($avar)) echo "$indent$var_name = <span style='color:#666666'>$type(".strlen($avar).")</span> $type_color".($avar == 1 ? "TRUE":"FALSE")."</span><br>";
 elseif(is_null($avar)) echo "$indent$var_name = <span style='color:#666666'>$type(".strlen($avar).")</span> {$type_color}NULL</span><br>";
 else echo "$indent$var_name = <span style='color:#666666'>$type(".strlen($avar).")</span> ".htmlentities($avar)."<br>";
 $var = $var[$keyvar];
 }
 
 echo "</div>";
}
?>

var_dump(get_defined_vars());
will dump all defined variables to the browser.

Be careful this outputs to stdout stream (1) instead of the proper stderr stream (2).

made 2 nifty functions based of what some people contributed here. Hope you find them usefull
usage ... call for the dump function. EG: dump($array, "Array dump");
<?php
////////////////////////////////////////////////////////
// Function: dump
// Inspired from: PHP.net Contributions
// Description: Helps with php debugging
function dump(&$var, $info = FALSE)
{
 $scope = false;
 $prefix = 'unique';
 $suffix = 'value';
 
 if($scope) $vals = $scope;
 else $vals = $GLOBALS;
 $old = $var;
 $var = $new = $prefix.rand().$suffix; $vname = FALSE;
 foreach($vals as $key => $val) if($val === $new) $vname = $key;
 $var = $old;
 echo "<pre style='margin: 0px 0px 10px 0px; display: block; background: white; color: black; font-family: Verdana; border: 1px solid #cccccc; padding: 5px; font-size: 10px; line-height: 13px;'>";
 if($info != FALSE) echo "<b style='color: red;'>$info:</b><br>";
 do_dump($var, '$'.$vname);
 echo "</pre>";
}
////////////////////////////////////////////////////////
// Function: do_dump
// Inspired from: PHP.net Contributions
// Description: Better GI than print_r or var_dump
function do_dump(&$var, $var_name = NULL, $indent = NULL, $reference = NULL)
{
 $do_dump_indent = "<span style='color:#eeeeee;'>|</span> &nbsp;&nbsp; ";
 $reference = $reference.$var_name;
 $keyvar = 'the_do_dump_recursion_protection_scheme'; $keyname = 'referenced_object_name';
 if (is_array($var) && isset($var[$keyvar]))
 {
 $real_var = &$var[$keyvar];
 $real_name = &$var[$keyname];
 $type = ucfirst(gettype($real_var));
 echo "$indent$var_name <span style='color:#a2a2a2'>$type</span> = <span style='color:#e87800;'>&amp;$real_name</span><br>";
 }
 else
 {
 $var = array($keyvar => $var, $keyname => $reference);
 $avar = &$var[$keyvar];
 
 $type = ucfirst(gettype($avar));
 if($type == "String") $type_color = "<span style='color:green'>";
 elseif($type == "Integer") $type_color = "<span style='color:red'>";
 elseif($type == "Double"){ $type_color = "<span style='color:#0099c5'>"; $type = "Float"; }
 elseif($type == "Boolean") $type_color = "<span style='color:#92008d'>";
 elseif($type == "NULL") $type_color = "<span style='color:black'>";
 
 if(is_array($avar))
 {
 $count = count($avar);
 echo "$indent" . ($var_name ? "$var_name => ":"") . "<span style='color:#a2a2a2'>$type ($count)</span><br>$indent(<br>";
 $keys = array_keys($avar);
 foreach($keys as $name)
 {
 $value = &$avar[$name];
 do_dump($value, "['$name']", $indent.$do_dump_indent, $reference);
 }
 echo "$indent)<br>";
 }
 elseif(is_object($avar))
 {
 echo "$indent$var_name <span style='color:#a2a2a2'>$type</span><br>$indent(<br>";
 foreach($avar as $name=>$value) do_dump($value, "$name", $indent.$do_dump_indent, $reference);
 echo "$indent)<br>";
 }
 elseif(is_int($avar)) echo "$indent$var_name = <span style='color:#a2a2a2'>$type(".strlen($avar).")</span> $type_color$avar</span><br>";
 elseif(is_string($avar)) echo "$indent$var_name = <span style='color:#a2a2a2'>$type(".strlen($avar).")</span> $type_color\"$avar\"</span><br>";
 elseif(is_float($avar)) echo "$indent$var_name = <span style='color:#a2a2a2'>$type(".strlen($avar).")</span> $type_color$avar</span><br>";
 elseif(is_bool($avar)) echo "$indent$var_name = <span style='color:#a2a2a2'>$type(".strlen($avar).")</span> $type_color".($avar == 1 ? "TRUE":"FALSE")."</span><br>";
 elseif(is_null($avar)) echo "$indent$var_name = <span style='color:#a2a2a2'>$type(".strlen($avar).")</span> {$type_color}NULL</span><br>";
 else echo "$indent$var_name = <span style='color:#a2a2a2'>$type(".strlen($avar).")</span> $avar<br>";
 $var = $var[$keyvar];
 }
}
?>

Howdy!
I am working on a pretty large project where I needed to dump a human readable form of whatever into the log files... and I thought var_export was too difficult to read. BigueNique at yahoo dot ca has a nice solution, although I needed to NOT modify whatever was being passed to dump.
So borrowing heavily from BigueNique's (just reworked his function) and someone's idea over in the object cloning page, I came up with the following function.
It makes a complete copy of whatever object you initially pass it, including all recursive definitions and outside objects references, then does the same thing as BigueNique's function. I also heavily reworked what it output, to suit my needs.
<?php
function var_log(&$varInput, $var_name='', $reference='', $method = '=', $sub = false) {
 static $output ;
 static $depth ;
 if ( $sub == false ) {
 $output = '' ;
 $depth = 0 ;
 $reference = $var_name ;
 $var = serialize( $varInput ) ;
 $var = unserialize( $var ) ;
 } else {
 ++$depth ;
 $var =& $varInput ;
 
 }
 
 // constants
 $nl = "\n" ;
 $block = 'a_big_recursion_protection_block';
 
 $c = $depth ;
 $indent = '' ;
 while( $c -- > 0 ) {
 $indent .= '| ' ;
 }
 // if this has been parsed before
 if ( is_array($var) && isset($var[$block])) {
 
 $real =& $var[ $block ] ;
 $name =& $var[ 'name' ] ;
 $type = gettype( $real ) ;
 $output .= $indent.$var_name.' '.$method.'& '.($type=='array'?'Array':get_class($real)).' '.$name.$nl;
 
 // havent parsed this before
 } else {
 // insert recursion blocker
 $var = Array( $block => $var, 'name' => $reference );
 $theVar =& $var[ $block ] ;
 // print it out
 $type = gettype( $theVar ) ;
 switch( $type ) {
 
 case 'array' :
 $output .= $indent . $var_name . ' '.$method.' Array ('.$nl;
 $keys=array_keys($theVar);
 foreach($keys as $name) {
 $value=&$theVar[$name];
 var_log($value, $name, $reference.'["'.$name.'"]', '=', true);
 }
 $output .= $indent.')'.$nl;
 break ;
 
 case 'object' :
 $output .= $indent.$var_name.' = '.get_class($theVar).' {'.$nl;
 foreach($theVar as $name=>$value) {
 var_log($value, $name, $reference.'->'.$name, '->', true);
 }
 $output .= $indent.'}'.$nl;
 break ;
 
 case 'string' :
 $output .= $indent . $var_name . ' '.$method.' "'.$theVar.'"'.$nl;
 break ;
 
 default :
 $output .= $indent . $var_name . ' '.$method.' ('.$type.') '.$theVar.$nl;
 break ;
 
 }
 
 // $var=$var[$block];
 
 }
 
 -- $depth ;
 
 if( $sub == false )
 return $output ;
 
}
// var_log( $var, '$name' ) ;
?>

Hope it works well for you!

Note that var_dump reset()'s array internal pointer!

If you want to save exactly the content of an array into a variable to save ir later for example, use this:
<?php $xml = var_export($xml, true); ?>

You'll have in your same variable the content readable, instead of written down or anything else

As Bryan said, it is possible to capture var_dump() output to a string. But it's not quite exact if the dumped variable contains HTML code.
You can use this instead:
<?php
echo '<pre>'; // This is for correct handling of newlines
ob_start();
var_dump($var);
$a=ob_get_contents();
ob_end_clean();
echo htmlspecialchars($a,ENT_QUOTES); // Escape every HTML special chars (especially > and < )
echo '</pre>';
?>

You can also use the PEAR package available at http://pear.php.net/package/Var_Dump
which parses the variable content in a very pleasant manner, a lot more easier to "follow" than the built-in var_dump() function. 
Of course there are many others, but I prefer this one, because it's simply to use.
Just add at the begining of your file:
<?php
require('Var_Dump.php'); // make sure the pear package path is set in php.ini
Var_Dump::displayInit(array('display_mode' => 'HTML4_Text'), array('mode' => 'normal','offset' => 4));
?>

then, instead of simply using var_dump($foo), use:
<?php
Var_Dump::display($foo);
?>

Read the documentation if you're looking for different output layouts.
Cheers!
Vladimir Ghetau

I wrote this dandy little function for using var_dump() on HTML documents so I don't have to view the source.
<?php
function htmldump($variable, $height="9em") {
echo "<pre style=\"border: 1px solid #000; height: {$height}; overflow: auto; margin: 0.5em;\">";
var_dump($variable);
echo "</pre>\n";
}
?>

You can pass arguments like this:
<?php
$sql = mysql_query("SELECT id, name, value FROM table WHERE cond = 'value'");
$s = mysql_fetch_assoc($sql);
// Dump variable containing the result of the MySQL query
var_dump($s);
?>

The second parameter lets you specify the height of the box. Default is 9em, but if you're expecting a huge output you'll probably want a higher value.
<?php
var_dump($s, "17em");
?>

Happy var_dumping.

It's important to note that the output depends on the precision directive.
<?php
var_dump(10000000000000.5); // float(10000000000000)
ini_set('precision',-1);
var_dump(10000000000000.5); // float(10000000000000.5)
?>