Description:
------------
---
From manual page: https://php.net/language.references.pass
---

Usually, you must define a variable before it is used, otherwise PHP will produce a notice.

When an argument defined as pass-by-reference though, defining the variable before using it is no longer neccessary.

This is not mentioned in the pass-by-ref docs, nor in the function argument docs:
https://www.php.net/manual/en/functions.arguments.php#functions.arguments.by-reference


Test script:
---------------
function output($s) {
    echo $s;
}
output($a); // => PHP emits notice, "Undefined variable: a"
$a = "Test";
output($a); // => outputs "Test"

function outputByRef(&$s) {
    echo $s;
}
outputByRef($a); // => no output, no notice
$a = "Test";
outputByRef($a); // => outputs "Test"