what is the difference between these two statements in php?

in the second you use &$x and that's passing a variable by reference. it calls reference by value

so the $x out of the function will change if you do something in the function.

reference

here the final value of $a is 5:

$a = 5;
$y  = fn ($x) => $x++;
$y($a);
echo $a;//5

but in reference by value, the final value of $a changed and it's 6:

$a = 5;
$y  = fn (&$x) => $x++;
$y($a);
echo $a;//6

So you can see in reference by value, the original value of the variable can be changed

& sets a reference to the variable fn($x = 42) : is a function with one or 0 parameter, if 0, default value 42

fn(&$x) : is a function with one parameter passed as reference, meaning if changed in the function, the new value of $x will be pass in the main.