Check brackets for balance

check_brackets.php

<?php
/**
 * User: vyacheslav.demyanov
 * Check brackets for balance
 * [([]{[]})] — balanced, but {[}], [{)] and ]{}[ - not.
 */
if (isset($argv[1])) {
    $str = $argv[1];
} else {
    $str = '[([]{[]})]';
}
echo "Checking string: $str\n";
echo "Is balanced: ".(int)isBalanced($str)."\n";
function isBalanced($str) {
    $stack = [];
    for ($i = 0; $i < strlen($str); $i++) {
        if (in_array($str[$i], ['(', '[', '{'])) {
            $stack[] = $str[$i];
        } else {
            $last = array_pop($stack);
            if ($str[$i] != invert($last)) {
                return false;
            }
        }
    }
    return true;
}
function invert($bracket) {
    $inverse = false;
    switch ($bracket) {
        case '(':
            $inverse = ')';
            break;
        case '[':
            $inverse = ']';
            break;
        case '{':
            $inverse = '}';
            break;
    }
    return $inverse;
}