Yes, null == 0. (PHP is still loose typed language..)

So what should I give to $limit to explode unlimited ???... As I know - in PHP no parameter means some default value - what default value is defined to default $limit ?..

now I have to write..

if ($lim === null) {
    $parts = explode(',', $str);
} else {
    $parts = explode(',', $str, $lim);
}

..instead of..

$parts = explode(',', $str, $lim);

Isn't it some sort of crap ?..

No, it's how the language works. 

while null does evaluate to 0 when compared, most other functions treat a NULL valued argument as though the argument did not exist, i.e. NULL == DEFAULT. The behaviour of explode to treat NULL as 0 comes back to bite us when we want to create a function that duplicates and extends explode's functionality. Consider the following:

function explode_and_trim($separator, $string, $limit = null)
{
  $a = explode($separator, $string, $limit);
  foreach ($a as $k => $v) {
    $a[$k] = trim($v);
  }
  return $a;
}

In that scenario, I cannot tell explode that there is NO limit unless I add a ridiculously unnecessary IF block to decide if it should or should not be called with the limit argument. NULL should be NULL, and regardless of whether PHP is loose typed or not, one can quite accurately test whether something is actually null (is_null($n)) or zero ($n === 0)

This is definitely a bug. 
16 years and this issue is still there.

The problem is this has not much to do with the loosely typing of PHP.
if a parameter defaults to null and you pass on a value of null you expect it to work the same.

Explode however treats the default value of the $limit parameter, which is null, different than a passed on value of null.

This might have been incorrectly documented at some earlier point in time, but if you check the current documentation at https://www.php.net/explode, you'll find that the default value for $limit is PHP_INT_MAX, not null. If you want to replicate the default behavior, that is the value that you need to pass.

Null is very much not a "use the default" value. This is only true for parameters that use null as default (duh).

Ah sorry about that, thanks for the quick reply, in that case it definitely makes sense and it's as expected.

I should have checked the php.net page first, it seems the latest version of PHPStorm still has the wrong documentation stating the $limit parameter defaults to null. I'll try to notify them about it.

A short follow up on my previous comment;
They fixed it in the latest PHPStorm early access build. ^_^