Declaration of Methods should be Compatible with Parent Methods in PHP

Strict Standards: Declaration of childClass::customMethod() should be compatible with that of parentClass::customMethod()

What are possible causes of this error in PHP? Where can I find information about what it means to be compatible?

childClass::customMethod() has different arguments, or a different access level (public/private/protected) than parentClass::customMethod().

This message means that there are certain possible method calls which may fail at run-time. Suppose you have

class A { public function foo($a = 1) {;}}
class B extends A { public function foo($a) {;}}
function bar(A $a) {$a->foo();}

The compiler only checks the call $a->foo() against the requirements of A::foo() which requires no parameters. $a may however be an object of class B which requires a parameter and so the call would fail at runtime.

This however can never fail and does not trigger the error

class A { public function foo($a) {;}}
class B extends A { public function foo($a = 1) {;}}
function bar(A $a) {$a->foo();}

So no method may have more required parameters than its parent method.

The same message is also generated when type hints do not match, but in this case PHP is even more restrictive. This gives an error:

class A { public function foo(StdClass $a) {;}}
class B extends A { public function foo($a) {;}}

as does this:

class A { public function foo($a) {;}}
class B extends A { public function foo(StdClass $a) {;}}

That seems more restrictive than it needs to be and I assume is due to internals.

Visibility differences cause a different error, but for the same basic reason. No method can be less visible than its parent method.

if you wanna keep OOP form without turning any error off, you can also:

class A
{
    public function foo() {
        ;
    }
}
class B extends A
{
    /*instead of : 
    public function foo($a, $b, $c) {*/
    public function foo() {
        list($a, $b, $c) = func_get_args();
        // ...
        
    }
}

Just to expand on this error in the context of an interface, if you are type hinting your function parameters like so:

interface A

use Bar;

interface A
{
    public function foo(Bar $b);
}

Class B

class B implements A
{
    public function foo(Bar $b);
}

If you have forgotten to include the use statement on your implementing class (Class B), then you will also get this error even though the method parameters are identical.

I faced this problem while trying to extend an existing class from GitHub. I'm gonna try to explain myself, first writing the class as I though it should be, and then the class as it is now.

What I though

namespace mycompany\CutreApi;

use mycompany\CutreApi\ClassOfVendor;

class CutreApi extends \vendor\AwesomeApi\AwesomeApi
{
   public function whatever(): ClassOfVendor
   {
        return new ClassOfVendor();
   }
}

What I've finally done

namespace mycompany\CutreApi;

use \vendor\AwesomeApi\ClassOfVendor;

class CutreApi extends \vendor\AwesomeApi\AwesomeApi
{
   public function whatever(): ClassOfVendor
   {
        return new \mycompany\CutreApi\ClassOfVendor();
   }
}

So seems that this errror raises also when you're using a method that return a namespaced class, and you try to return the same class but with other namespace. Fortunately I have found this solution, but I do not fully understand the benefit of this feature in php 7.2, for me it is normal to rewrite existing class methods as you need them, including the redefinition of input parameters and / or even behavior of the method.

One downside of the previous aproach, is that IDE's could not recognise the new methods implemented in \mycompany\CutreApi\ClassOfVendor(). So, for now, I will go with this implementation.

Currently done

namespace mycompany\CutreApi;

use mycompany\CutreApi\ClassOfVendor;

class CutreApi extends \vendor\AwesomeApi\AwesomeApi
{
   public function getWhatever(): ClassOfVendor
   {
        return new ClassOfVendor();
   }
}

So, instead of trying to use "whatever" method, I wrote a new one called "getWhatever". In fact both of them are doing the same, just returning a class, but with diferents namespaces as I've described before.

Hope this can help someone.