Why Can\'t I Call Non-Static PHP Methods Using the Double Colon (::) Operator?

Understanding Non-Static Method Invocation with Double Colon (::)

When attempting to call a non-static method using the syntax of static methods (class::method), you may encounter an error. Contrary to being a configuration issue, this behavior is intrinsic to PHP's design.

In PHP, non-static methods require an instance to be created first in order to be invoked. The error occurs because when using class::method syntax, no instance is explicitly provided.

Distinction Between Static and Non-Static Methods

Static methods can be called without an instance of the class, while non-static methods require an instance. This distinction is evident in the example below:

class Teste {

    public function fun1() {
        echo 'fun1';
    }
    public static function fun2() {
        echo "static fun2" ;
    }
}

Teste::fun2(); // This is valid because fun2 is a static method
Teste::fun1(); // This will generate an error because fun1 is not a static method

Inconsistent Behavior

However, PHP exhibits some inconsistent behavior with non-static methods called statically. If a non-static method is called statically from within a non-static method of the same class, $this within the non-static method will refer to the current instance of the class.

class A 
{
    public function test()
    {
        echo $this->name;
    }
}

class C 
{
    public function q()
    {
        $this->name = 'hello';
        A::test();
    }
}

$c = new C;
$c->q(); // This will print 'hello'

This behavior can be considered erroneous with strict error reporting enabled.

Conclusion

Calling non-static methods using the syntax of static methods is generally discouraged. It can lead to unexpected behavior or errors. Instead, it's recommended to use the proper syntax for invoking non-static methods, which involves creating an instance of the class and then calling the method on that instance.

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