Feature detection of classes or objects

class MyClass {
    public $public_field;
    protected $protected_field;
    private $private_field;
    static $static_field;
    const CONSTANT = 0;
    public function public_function() {}
    protected function protected_function() {}
    private function private_function() {}
    static function static_function() {}
}

// check properties
$check = property_exists('MyClass', 'public_field');    // true
$check = property_exists('MyClass', 'protected_field'); // true
$check = property_exists('MyClass', 'private_field');   // true, as of PHP 5.3.0
$check = property_exists('MyClass', 'static_field');    // true
$check = property_exists('MyClass', 'other_field');     // false

// check methods
$check = method_exists('MyClass', 'public_function');    // true
$check = method_exists('MyClass', 'protected_function');    // true
$check = method_exists('MyClass', 'private_function');    // true
$check = method_exists('MyClass', 'static_function');    // true

// however...
$check = property_exists('MyClass', 'CONSTANT');  // false
$check = property_exists($object, 'CONSTANT');    // false

$r = new ReflectionClass('MyClass');
$check = $r->hasProperty('public_field');  // true
$check = $r->hasMethod('public_function'); // true
$check = $r->hasConstant('CONSTANT');      // true
// also works for protected, private and/or static members.