Silence "Declaration ... should be compatible" warnings in PHP 7

Question

After upgrade to PHP 7 the logs almost choked on this kind of errors:

PHP Warning: Declaration of Example::do($a, $b, $c) should be compatible with ParentOfExample::do($c = null) in Example.php on line 22548

How do I silence these and only these errors in PHP 7?

• Before PHP 7 they were E_STRICT type of warnings which could be easily dealt with. Now they're just plain old warnings. Since I do want to know about other warnings, I can't just turn off all warnings altogether.

• I don't have a mental capacity to rewrite these legacy APIs not even mentioning all the software that uses them. Guess what, nobody's going to pay for that too. Neither I develop them in the first place so I'm not the one for blame. (Unit tests? Not in the fashion ten years ago.)

• I would like to avoid any trickery with func_get_args and similar as much as possible.

• Not really I want to downgrade to PHP 5.

• I still want to know about other errors and warnings.

Is there a clean and nice way to accomplish this?

Answer 1

1. Workaround

Since it is not always possible to correct all the code you did not write, especially the legacy one...

if (PHP_MAJOR_VERSION >= 7) {
    set_error_handler(function ($errno, $errstr) {
       return strpos($errstr, 'Declaration of') === 0;
    }, E_WARNING);
}

This error handler returns true for warnings beginning with Declaration of which basically tells PHP that a warning was taken care of. That's why PHP won't report this warning elsewhere.

Plus, this code will only run in PHP 7 or higher.

---

If you want this to happen only in regard to a specific codebase, then you could check if a file with an error belongs to that codebase or a library of interest:

if (PHP_MAJOR_VERSION >= 7) {
    set_error_handler(function ($errno, $errstr, $file) {
        return strpos($file, 'path/to/legacy/library') !== false &&
            strpos($errstr, 'Declaration of') === 0;
    }, E_WARNING);
}

---

2. Proper solution

As for actually fixing someone else's legacy code, there is a number of cases where this could be done between easy and manageable. In examples below class B is a subclass of A. Note that you do not necessarily will remove any LSP violations by following these examples.

1. Some cases are pretty easy. If in a subclass there's a missing default argument, just add it and move on. E.g. in this case:

   Declaration of B::foo() should be compatible with A::foo($bar = null)
   

   You would do:

   - public function foo()
   + public function foo($bar = null)
   

2. If you have additional constrains added in a subclass, remove them from the definition, while moving inside the function's body.

   Declaration of B::add(Baz $baz) should be compatible with A::add($n)
   

   You may want to use assertions or throw an exception depending on a severity.

   - public function add(Baz $baz)
   + public function add($baz)
     {
   +     assert($baz instanceof Baz);
   

   If you see that the constraints are being used purely for documentation purposes, move them where they belong.

   - protected function setValue(Baz $baz)
   + /**
   +  * @param Baz $baz
   +  */
   + protected function setValue($baz)
     {
   +     /** @var $baz Baz */
   

3. If you subclass has less arguments than a superclass, and you could make them optional in the superclass, just add placeholders in the subclass. Given error string:

   Declaration of B::foo($param = '') should be compatible with A::foo($x = 40, $y = '')
   

   You would do:

   - public function foo($param = '')
   + public function foo($param = '', $_ = null)
   

4. If you see some arguments made required in a subclass, take the matter in your hands.

   - protected function foo($bar)
   + protected function foo($bar = null)
     {
   +     if (empty($bar['key'])) {
   +         throw new Exception("Invalid argument");
   +     }
   

5. Sometimes it may be easier to alter the superclass method to exclude an optional argument altogether, falling back to func_get_args magic. Do not forget to document the missing argument.

     /**
   +  * @param callable $bar
      */
   - public function getFoo($bar = false)
   + public function getFoo()
     {
   +     if (func_num_args() && $bar = func_get_arg(0)) {
   +         // go on with $bar
   

   Sure this can become very tedious if you have to remove more than one argument.

6. Things get much more interesting if you have serious violations of substitution principle. If you do not have typed arguments, then it is easy. Just make all extra arguments optional, then check for their presence. Given error:

   Declaration of B::save($key, $value) should be compatible with A::save($foo = NULL)
   

   You would do:

   - public function save($key, $value)
   + public function save($key = null, $value = null)
     {
   +     if (func_num_args() < 2) {
   +         throw new Exception("Required argument missing");
   +     }
   

   Note that we couldn't use func_get_args() here because it does not account for default (non-passed) arguments. We are left with only func_num_args().

7. If you have a whole hierarchies of classes with a diverging interface, it may be easier diverge it even further. Rename a function with conflicting definition in every class. Then add a proxy function in a single intermediary parent for these classes:

   function save($arg = null) // conforms to the parent
   {
       $args = func_get_args();
       return $this->saveExtra(...$args); // diverged interface
   }
   

   This way LSP would still be violated, although without a warning, but you get to keep all type checks you have in subclasses.

Answer 2

For those who want to actually correct your code so it no longer triggers the warning: I found it useful to learn that you can add additional parameters to overridden methods in subclasses as long as you give them default values. So for example, while this will trigger the warning:

//"Warning: Declaration of B::foo($arg1) should be compatible with A::foo()"
class B extends A {
    function foo($arg1) {}
}

class A {
    function foo() {}
}

This will not:

class B extends A {
    function foo($arg1 = null) {}
}

class A {
    function foo() {}
}

Answer 3

If you must silence the error, you can declare the class inside a silenced, immediately-invoked function expression:

<?php

// unsilenced
class Fooable {
    public function foo($a, $b, $c) {}
}

// silenced
@(function () {
    class ExtendedFooable extends Fooable {
        public function foo($d) {}
    }
})();

I would strongly recommend against this, though. It is better to fix your code than to silence warnings about how it is broken.

---

If you need to maintain PHP 5 compatibility, be aware that the above code only works in PHP 7, because PHP 5 did not have uniform syntax for expressions. To make it work with PHP 5, you would need to assign the function to a variable before invoking it (or make it a named function):

$_ = function () {
    class ExtendedFooable extends Fooable {
        public function foo($d) {}
    }
};
@$_();
unset($_);

Answer 4

PHP 7 removes the E_STRICT error level. Info about this can be found in the PHP7 compatibility notes. You might also want to read the proposal document where it was discussed while PHP 7 was being developed.

The simple fact is this: The E_STRICT notices were introduced a number of versions ago, in an attempt to notify developers that they were using bad practice, but initially without trying to force any changes. However recent versions, and PHP 7 in particular, have become more strict about these things.

The error you're experiencing is a classic case:

You have defined a method in your class that overrides a method of the same name in the parent class, but your override method has a different argument signature.

Most modern programming languages would not actually allow this at all. PHP used to allow developers to get away with stuff like this, but the language is becoming more strict with every version, especially now with PHP 7 -- they went with a new major version number specifically so that they could justify making significant changes that break backward compatibility.

The problem you have is because you've already been ignoring the warning messages. Your question implies that this is the solution you want to continue with, but messages like "strict" and "deprecated" should be treated as an explicit warning that your code is likely to break in future versions. By ignoring them for the past number of years, you have effectively placed yourself in the situation you have now. (I know that's not what you want to hear, and doesn't really help the situation now, but it's important to make it clear)

There really isn't a work around of the kind you're looking for. The PHP language is evolving, and if you want to stick with PHP 7 your code will need to evolve too. If you really can't fix the code, then you will either have to suppress all warnings or else live with these warnings cluttering up your logs.

The other thing you need to know if you plan to stick with PHP 7 is that there are a number of other compatibility breaks with this version, including some that are quite subtle. If your code is in a state where it has errors like the one you're reporting, it means that it's probably been around for quite a while, and likely has other issues that will cause you problems in PHP 7. For code like this, I would suggest doing a more thorough audit of the code before committing to PHP 7. If you're not prepared to do that, or not prepared to fix the bugs that are found (and the implication from your question is that you are not), then I'd suggest that PHP 7 is probably an upgrade too far for you.

You do have the option of reverting to PHP 5.6. I know you said you don't want to do that, but as a short-to-medium term solution it will make things easier for you. Frankly, I think it might be your best option.