Checking Balanced Parentheses (an Application of Stacks)

In this lesson you will apply a stack to check that brackets match up, seeing a data structure put to practical use. This is for anyone searching for "PHP balanced parentheses check".

With a stack (last in, first out) you can push opening brackets such as ( and [, then check each closing bracket against whatever is on top. That is how bracket matching works.

The sample code uses the $stack array as a stack: array_push() on an opening bracket, and on a closing one it checks the value from array_pop() against the $pairs associative array to confirm it is the right partner.

A common stumbling block for beginners is the case where the closing bracket is the wrong kind. Matching counts alone is not enough - you have to confirm that each pair is of the right type, and that is the heart of this algorithm.

The same idea is used behind editor auto-completion and code syntax checking. Plenty of similar tools check that HTML opening and closing tags line up correctly.

📖 Reference code

<?php
function isBalanced($str) {
    $stack = [];
    $pairs = [")" => "(", "]" => "[", "}" => "{"];
    for ($i = 0; $i < strlen($str); $i++) {
        $ch = $str[$i];
        if (in_array($ch, ["(", "[", "{"])) {
            array_push($stack, $ch);
        } elseif (in_array($ch, [")", "]", "}"])) {
            if (empty($stack) || array_pop($stack) !== $pairs[$ch]) {
                return false;
            }
        }
    }
    return empty($stack);
}

var_dump(isBalanced("(a[b]{c})"));
var_dump(isBalanced("(a[b)]"));

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🧪 This site can't compile or run PHP directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).