Fixed in CVS. Will be available in 0.18.0a3.

If you want to patch your version add the following code to the end of core/php_api.php:

# array_key_exists was not available on PHP 4.0.6
if ( ! function_exists( 'array_key_exists' ) ) {
    function array_key_exists( $key, $search ) {
        return key_exists( $key, $search );
    }
}