Function php meaning

It really is not a function. It is an expression that works with a comparison operator ? called Ternary Operator .

return $a < $b ? -1 : 1;

Its interpretation is similar to this:

// si $a es menor que $b...
if($a < $b) {
    return -1;
}
else {
    return 1;
}

On the contrary that happens with the if , where it is habitual and behaves perfectly, it is not recommended to nest or stack several expressions.

$a < $b ? -1 : $c ? -2 : 1;

For such a case, the parentheses should be used to establish preferences and obtain the desired result or opt for a structure with a more evident reading to avoid errors. As for example if .

It's a compact, simplified if . What it does is a check of $a is less than $b if it is, returns -1 and otherwise 1

let's say the structure <comparación> ? cierto : falso;

Another way to do it would be like this

$r = 1;
if ($a < $b) $r= -1;
return $r;