Longest Valid Parentheses
<p>Given a string containing just the characters <code>&#39;(&#39;</code> and <code>&#39;)&#39;</code>, return <em>the length of the longest valid (well-formed) parentheses </em><span data-keyword="substring-nonempty"><em>substring</em></span>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;(()&quot;
<strong>Output:</strong> 2
<strong>Explanation:</strong> The longest valid parentheses substring is &quot;()&quot;.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;)()())&quot;
<strong>Output:</strong> 4
<strong>Explanation:</strong> The longest valid parentheses substring is &quot;()()&quot;.
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;&quot;
<strong>Output:</strong> 0
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>0 &lt;= s.length &lt;= 3 * 10<sup>4</sup></code></li>
	<li><code>s[i]</code> is <code>&#39;(&#39;</code>, or <code>&#39;)&#39;</code>.</li>
</ul>
Stack; String; Dynamic Programming
class Solution {
    /**
     * @param string $s
     * @return integer
     */

    function longestValidParentheses($s) {
        $stack = [];
        $maxLength = 0;

        array_push($stack, -1);
        for ($i = 0; $i < strlen($s); $i++) {
            if ($s[$i] === '(') {
                array_push($stack, $i);
            } else {
                array_pop($stack);

                if (empty($stack)) {
                    array_push($stack, $i);
                } else {
                    $length = $i - end($stack);
                    $maxLength = max($maxLength, $length);
                }
            }
        }
        return $maxLength;
    }
}
