Quick search showed that there is no well know scripting language that has such function. C++/C# has this but they are not scripting languages.
The following code does the same. Also we have to think about BC(backward compatibility) with older scripts.
function sign($x){
 return (int)((abs($x)-$x)? -1:$x>0);
}

Thank you for you suggestion.

I also searched for a sign() function and ended up here. But I disagree with Andrey's arguments not to include it into PHP.

Since when is it a policy to *not* include a function, simply because it does not exist in a number of other languages? Doesn't PHP have its own 'vision'? Secondly, a new function should not break BC.

As Bill stated, it would complement the abs() function and make the Math list a more complete list, even though it's a simple function. It seems a better idea to me to add "sign()" then let's say "goto".

Seconded. I've always rolled my own in PHP, but this is typically used in small loops executed a lot, so moving this logic into the PHP core would probably make a difference. If you're concerned sign() might already be used by various existing scripts, why not implement is as the more math-like sgn()?

Patrick, adding a new library function _absolutely_ breaks BC.

For example:

<?php
function abs($x) {
   return 3;
}
// Output: Fatal error: Cannot redeclare abs()
?>

Do you want every script that declares a function `sign` to suddenly not work any more? How would this *not* be a compatibility issue?

sign is very usefull in usort function
exemple :

<?php
$people = [
    ["name"=>"John","age"=>20],
    ["name"=>"Jack","age"=>30],
    ["name"=>"Paul","age"=>25],
]

usort($people,function($a, $b){
     return sign($a['age'] - $b['age']);
});
?>

For usort() this is not needed. function($a,$b) { return $a['age'] - $b['age']; } is fine.