What causes E_STRICT errors in PHP

Some months ago this question came up on my work. We asked our self what kind of errors causes an E_STRICT error. The naive approach was to search the php.net. But I didn’t find anything about that. Yes there is some information what number E_STRICT has and that it causes errors if you don’t satisfy the strict standards and it’s important to be future compatible. But why? And what?

Then I thought: Ok, to implement this feature (E_STRICT standard) they needed a list for what they will throw an E_STRICT error. I asked Google. What i found was not really helpful. Only a not exhaustive list on pear.php.net. I wonder how they can implement something such E_STRICT, if they don’t have a list for that. Are they still hacking on without thinking about what to do?

So I gathered the sparely information from the web and started to grep the PHP source code. So I can present a hopefully exhaustive list what causes E_STRICT errors:

• when static methods are declared as abstract

• when non-static methods are called static

• when a static property is accessed dynamically

• when assigning new with reference

  $foo =& new Foo();

• if you use is_a() instead of instanceof (in 5.2, in 5.3 undeprecated…)

• if you use var instead of private/protected/public for class properties

• if method declaration/signature of overridden methods differ

• if you declare both the PHP4 and PHP5 constructor in a class

• if you use “Automagic Objects”

  $notInitializedYet->foo = ‘bar’;

• if you assign function/method return values as reference

  $foo =& bar();

• if you pass function/method return values as reference to a function/method

  foo(bar()); //if function foo(&$aReference { … }

• if resources are casted implicit to an integer

  $row[$query_id] = mysql_fetch_array($query_id); // if get_type($query_id) == ‘resource’

• if mktime() is called without parameter. You should use time() instead

• if you call mysqli_next_result()/mysqli::next_result() although there are no further results. You should test with mysqli_more_results()/mysqli::more_results() before

• and last but not least: If you don’t set a default timezone (neither in Code nor in php.ini)