Palindrome in PHP

Palindrome.php

<?php

function isPalindrome( $s ) {
	$i = 0;
	$j = strlen( $s ) - 1;

	while ( $i < $j ) {
		if ( $s[ $i ] !== $s[ $j ] ) {
			return false;
		}

		$j --;
		$i ++;
	}

	return true;
}

function countPalindromeSubStrings( $s ) {
	$i = 0;
	$palindromes = [];
	$s = implode( '|', str_split( $s ) );

	while ( $i < strlen( $s ) ) {

		if ( ! isset( $palindromes[ $s[ $i ] ] ) && $s[ $i ] !== '|' ) {
			$palindromes[ $s[ $i ] ] = true;
		}

		$j = $i - 1;
		$k = $i + 1;

		$palindrome = '';

		while ( $k < strlen( $s ) ) {

			if ( $j < 0 || $k > strlen( $s ) ) {
				break;
			}

			if ( ! $palindrome && $s[ $i ] !== '|' ) {
				$palindrome = $s[ $i ];
			}

			if ( $s[ $j ] === $s[ $k ] && $s[ $j ] !== '|' && $s[ $k ] !== '|' ) {
				$palindrome = $s[ $j ] . $palindrome . $s[ $k ];

				if ( ! isset( $palindromes[ $palindrome ] ) ) {
					$palindromes[ $palindrome ] = true;
				}
			}

			$j --;
			$k ++;
		}

		$i ++;
	}

	return $palindromes;
}

//var_dump( countPalindromeSubStrings( 'abaaba' ) );
isPalindrome( 'aa' );