'x' is not allowed as parameter type (allowed only 'array' or class name)

Sample code:

// ...
class y {
    function m(string param) {     // 'string' is not allowed as parameter type (allowed only 'array' or class name)
        //...
    }
    //...
}
//...

This code would issue message "'string' is not allowed as parameter type (allowed only array or class name)". In PHP you can't declare parameter to function to be any simple type.