func_num_args

Just a note for anyone wondering. This function doesn't include params that have a default value, unless you pass one in to overwrite the default param value. Not sure if that makes sense, so here's an example:
<?php
function helloWorld($ArgA, $ArgB="HelloWorld!") {
 return func_num_args();
}
// The following will return 1
$Returns1 = helloWorld("HelloWorld!");
// The following will return 2
$Returns2 = helloWorld("HelloWorld!", "HowdyWorld!");
?>

This function comes in handy, and I believe is the only solution, when you have an optional parameter that can take any type of data.
For example:
<?php
// $data can be of any type, including null
function my_function($name, $data = null)
{
 if ($data !== null)
 {
 // Do something with $data
 // If you call my_function('something'), this WILL NOT be reached
 // If you call my_function('something', null), this WILL NOT be reached
 }
}
?>

The problem with the above function is that you will never be able to use null as the value for $data. To fix this, use func_num_args() like so:
<?php
// $data can be of any type, including null
function my_function($name, $data = null)
{
 if (func_num_args() >= 2)
 {
 // Do something with $data
 // If you call my_function('something'), this WILL NOT be reached
 // If you call my_function('something', null), this WILL be reached
 }
}
?>

This solution works because func_num_args() reports exactly how many arguments were passed when the function was called. It does not take into account when default argument values are used.

I had defined a function function_name(){ ...} as a drupal callback.
I try to get how many params where passed
I got a Error and my Site falls down
I've replaced func_get_args() instead func_num_args() and my Site was restored.
I conclude you can not use func_num_args() in callbacks.
Hope it helps.

The idea of func_get_args() is to construct functions of variable number of parameters like
<?php
function var_param_func(){
 if(func_num_args()==0){
 //do one thing
 }
 if(func_num_args()==1)
 //do another thing
 //get the args with func_get_args()
 }
}
?>

If you are using PHP 7 and func_num_args is in your base class which you extended, you can pass your arguments with the 'spat' operator.
class Sql {
public function doGetWhere(...$args) {
$num_args = func_num_args();
 $args_list = func_get_args();
 echo '<pre>';
 var_dump($args_list);
 echo '<pre>';
 }
}
class Member extends Sql {
public function getWhere(...$args) {
 $this->doGetWhere(...$args);
 }
}
$member = new Member();
$member->getWhere('first_name','last_name','userlevel','email','where','email','=',$sub_email);
However, take note that if you 'new up' the 'Sql' class in your 'Member' class above, instead of extending it, you will not need to pass your arguments as a variable. Just my two cents. -Bruce tong

If you want to pass the parameters on intact to another function, use func_get_args and call_user_func_array (careful - this one is only available in recent PHP versions). For example:
<?php
/* Print an HTML tag. This accepts a variable number of arguments:
 the first should be the name of the tag, followed by pairs of
 arguments that describe keys and values. The values are printed
 with surrounding double quote characters. */
function printTag() {
 $numArgs = func_num_args();
 if ($numArgs < 1) die("printTag given no arguments");
 
 echo "<" . func_get_arg(0);
 for ($i = 1; $i < $numArgs; $i+=2) {
 echo " " . func_get_arg($i);
 if ($i+1 < $numArgs)
 echo "=\"" . func_get_arg($i+1) . "\"";
 }
 echo ">";
 
}
/* Print an HTML tag with a newline on the end */
function printTagNL() {
 $args = func_get_args();
 call_user_func_array("printTag", $args);
 echo "\n";
}
printTagNL("input", "type", "hidden", "name", "SORTORDER", "value", $columnNo);
?>