Balanced Parentheses

Walk a string of bracket characters. Push every opening bracket. On a closing bracket, pop and verify it matches; mismatch or empty-stack pop means unbalanced. At end of string, stack must be empty.

Algorithm

Canonical input "({[]})" (balanced) finishes with the stack empty and the result true.

Basic Implementation

basic.php

<?php
function match_open($close) {
	if ($close == ')') {
		return '(';
	} else if ($close == ']') {
		return '[';
	} else if ($close == '}') {
		return '{';
	}
	return ' ';
}

$text = "({[]})";
$stack = [];
$balanced = true;
$i = 0;
while ($balanced && $i < strlen($text)) {
	$ch = $text[$i];
	if ($ch == '(' || $ch == '[' || $ch == '{') {
		$stack[] = $ch;
	} else {
		if (count($stack) == 0 || $stack[count($stack) - 1] != match_open($ch)) {
			$balanced = false;
		} else {
			array_pop($stack);
		}
	}
	$i = $i + 1;
}
if (count($stack) != 0) {
	$balanced = false;
}
if ($balanced) {
	echo "True\n";
} else {
	echo "False\n";
}

Complexity

Time: O(n)
Space: O(n) worst case

Implementation notes

PHP: a plain array with $stack[] = $ch; / array_pop($stack); / $stack[count($stack) - 1] is the smallest honest stack shape; reaching for SplStack would hide the explicit push/pop pattern. A $balanced flag plus while ($balanced && $i < strlen($text)) keeps the early-exit visible without leaning on break;.
The match_open helper documents the closing -> opening map without leaking runtime references into the replay.
strlen($text) is the canonical character count for an ASCII bracket string; the lesson does not exercise multibyte sequences.
The replay highlights the current character, shows the stack updating each frame, and surfaces the final balanced/unbalanced verdict.

stack push/pop Use a plain `$stack = []` array as the open-bracket stack; push by `$stack[] = $ch;`, pop by `array_pop($stack);`.

matching map A small `match_open($close)` helper returns the expected opener for each closing bracket — three explicit `if` arms keep the lesson compact.