<?php

    function byValue($a) {
        $a[] = 4;
    }

    function byRef(&$a) {
        $a[] = 5;
    }
    
    $a = array(1, 2, 3);

    byValue($a);
    print_r($a); // 1, 2, 3

    byRef($a);
    print_r($a); // 1, 2, 3, 5

Array
(
    [0] => 1
    [1] => 2
    [2] => 3
)
Array
(
    [0] => 1
    [1] => 2
    [2] => 3
    [3] => 5
)