When to Use `::` vs. `->` for Method Access in PHP?

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Deciphering :: (Double Colon) vs. -> (Arrow) in PHP

In PHP, accessing class methods can be done in two distinct ways: :: (double colon) and -> (arrow). Let's delve into the differences.

Scope and Usage

The primary factor determining the appropriate operator is the context in which it's used. For instance members, such as properties and non-static methods, -> is employed. Conversely, for class members, such as static properties and methods, :: is the preferred operator.

Syntactic Interpretation

When the left operand of -> is an object instance, it signifies that the method being accessed belongs to that instance. In contrast, :: suggests that the method is invoked on the class itself, rather than a specific instance.

Static Member Access

Generally, :: is used for static member access. However, in rare cases, :: can also be used to access instance members. An example is when accessing the parent class's implementation of an instance method from within the derived class.

Method Call Semantics

The semantics of the -> operator are more complex than those of ::. A call via -> results in an instance call if both the target method is not declared as static and there exists a compatible object context. Otherwise, it's treated as a static call.

Example

Consider the following code snippet:

class A {
    public $prop_instance;
    public function func_instance() {
        echo "in ", __METHOD__, "\n";
    }
}

class B extends A {
    public static $prop_static;
    public function func_static() {
        echo "in ", __METHOD__, "\n";
    }
}

$a = new A;
$b = new B;
echo '$a->prop_instance: ', $a->prop_instance, "\n";
echo 'B::$prop_static: ', B::$prop_static, "\n";
$b->func_instance();
B::func_static();

Output:

$a->prop_instance: 
B::$prop_static: 
in B::func_instance
in B::func_static

The consistent usage of -> for instance members and :: for static members ensures clarity and prevents ambiguous interpretations.

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