testFunction（（string）$ variable）; - 这个叫什么？

Question

I've seen some code written like this, and I'm really curious what it does and what it's for. Sorry for the unclear title, I appreciate all answers!

Edit: In particular I'm curious about the (string) $variable part

Answer 1

It's called type casting

Type casting in PHP works much as it does in C: the name of the desired type is written in parentheses before the variable which is to be cast.

<?php
$foo = 10;   // $foo is an integer
$bar = (boolean) $foo;   // $bar is a boolean
?>
The casts allowed are:

(int), (integer) - cast to integer
(bool), (boolean) - cast to boolean
(float), (double), (real) - cast to float
(string) - cast to string
(array) - cast to array
(object) - cast to object
(unset) - cast to NULL (PHP 5)

In your specific example a variable was being cast to a string before being passed as a parameter to testFunction()