random_int

Here is a simple backporting function, it works for PHP >= 5.1
<?php
if (!function_exists('random_int')) {
 function random_int($min, $max) {
 if (!function_exists('mcrypt_create_iv')) {
 trigger_error(
 'mcrypt must be loaded for random_int to work', 
 E_USER_WARNING
 );
 return null;
 }
 
 if (!is_int($min) || !is_int($max)) {
 trigger_error('$min and $max must be integer values', E_USER_NOTICE);
 $min = (int)$min;
 $max = (int)$max;
 }
 
 if ($min > $max) {
 trigger_error('$max can\'t be lesser than $min', E_USER_WARNING);
 return null;
 }
 
 $range = $counter = $max - $min;
 $bits = 1;
 
 while ($counter >>= 1) {
 ++$bits;
 }
 
 $bytes = (int)max(ceil($bits/8), 1);
 $bitmask = pow(2, $bits) - 1;
 
 if ($bitmask >= PHP_INT_MAX) {
 $bitmask = PHP_INT_MAX;
 }
 
 do {
 $result = hexdec(
 bin2hex(
 mcrypt_create_iv($bytes, MCRYPT_DEV_URANDOM)
 )
 ) & $bitmask;
 } while ($result > $range);
 
 return $result + $min;
 }
}
?>

Randomness test
<?php
$max = 100; // number of random values
$test = 1000000;
$array = array_fill(0, $max, 0);
for ($i = 0; $i < $test; ++$i) {
 ++$array[random_int(0, $max-1)];
}
function arrayFormatResult(&$item) {
 global $test, $max; // try to avoid this nowdays ;)
 
 $perc = ($item/($test/$max))-1;
 $item .= ' '. number_format($perc, 4, '.', '') .'%';
}
array_walk($array, 'arrayFormatResult');
print_r($array);
?>

On my system (5th gen i7) random_int() takes about 14x longer to do the same number of calculations than rand(). 
for ($i=0; $i<10000000; $i++)
 $x = rand(1, PHP_INT_MAX);
Takes 0.86 seconds for rand(), 12.29 seconds for random_int(). So use random_int() somewhat sparingly. 
If it's not really important to be truly random, use rand() instead, especially in a tight loop.

A small function for generating version 4 GUIDs.
<?php
function v4uuid()
{
 $a = str_pad(dechex(random_int(0x0000_0000, 0xffff_ffff)), '0', STR_PAD_LEFT);
 $b = str_pad(dechex(random_int(0x0000, 0xffff)), '0', STR_PAD_LEFT);
 $c = dechex(random_int(0x4000, 0x4fff));
 $d = dechex(random_int(0x8000, 0xbfff));
 $e = str_pad(dechex(random_int(0x0000_0000_0000, 0xffff_ffff_ffff)), '0', STR_PAD_LEFT);
 return "$a-$b-$c-$d-$e";
}
?>