Balanced braces

brace.php

<?php
define('OPENED_BRACE', '{');
define('CLOSED_BRACE', '}');
define('OPENED_PARENTHESIS', '(');
define('CLOSED_PARENTHESIS', ')');
define('OPENED_BRACKET', '[');
define('CLOSED_BRACKET', ']');


$values = [
	'() [] () ([]()[])',
	'( (] ([)]',
	'{}[]()',
	'{{}]}',
	'{[}',
	'[}]'
];

print_r(braces($values));

function braces($values)
{
	$result = [];
	
	foreach($values as $value) {
		$result[] = is_balanced($value);
	}
	
	return $result;
}

function is_balanced($string)
{
	$stack = [];
	
	$allowed_order = [
		CLOSED_BRACKET     => OPENED_BRACKET,
		CLOSED_PARENTHESIS => OPENED_PARENTHESIS,
		CLOSED_BRACE       => OPENED_BRACE
	];
	
	$allowed_closed_braces = array_keys($allowed_order);
	$allowed_opened_braces = $allowed_order;
	
	for($i = 0; $i < strlen($string); $i++) {
		$char = $string[$i];
		// add opening brace
		if(empty($stack) || in_array($char, $allowed_opened_braces)) {
			$stack[] = $char;
			continue;
		}
		// validate that only proper closing brace are allowed
		if(in_array($char, $allowed_closed_braces)) {
			$last_elemnt_in_stack = $stack[count($stack) - 1];
			if($last_elemnt_in_stack !== $allowed_order[$char]) {
				return "NO";
			} else {
			    array_pop($stack);
			}
		}
	}
	
	return "YES";
}