Decode String in PHP

DecodeString.php

<?php

class Solution {

    /**
     * @param String $s
     * @return String
     */
    function decodeString($s) {

		$stack = [];
		$num = '';
		$string = '';

		for( $i = 0; $i < strlen($s); $i ++ ) {

			if ( is_numeric( $s[$i] ) ) {
				$num .= $s[$i];
			} elseif ( ctype_alpha( $s[$i] ) ) {
				$string .= $s[$i];
			} elseif ( $s[$i] === '[' ) {
				$stack[] = [ 'num' => $num, 'string' => $string ];
				$num = '';
				$string = '';
			} else {
				$val = array_pop( $stack );
				$newString = '';
				for( $j = 0; $j < (int) $val['num']; $j ++ ) {
					$newString .= $string;
				}
				$string = $val['string'] . $newString;
			}

		}

		return $string;
	}
}