PHP method scope binding

This is the behavior in most languages (by design).

If a method is private, $this->method() will call it directly (ignore the derived class's method)

If a method is virtual (this concept comes from C++, in PHP and Java, all public/protected methods are virtual), $this->method() will call the derived class's method.

Rule is rule, it is an OOP concern. For example, in OOP we may want to make sure a method never got overridden, then we can make it private. If a developer overrides it by accident, and would hence introduce weird behaviour if the parent suddenly called the child's implementation. Thanks to @deceze

Rule: private and final methods on an object will always be called directly, without consulting the override table.

This rule is baked into the engine:

/* Check if this calls a known method on $this */
if (opline->op1_type == IS_UNUSED && opline->op2_type == IS_CONST &&
        CG(active_class_entry) && zend_is_scope_known()) {
    zend_string *lcname = Z_STR_P(CT_CONSTANT(opline->op2) + 1);
    fbc = zend_hash_find_ptr(&CG(active_class_entry)->function_table, lcname);

    /* We only know the exact method that is being called if it is either private or final.
     * Otherwise an overriding method in a child class may be called. */
    if (fbc && !(fbc->common.fn_flags & (ZEND_ACC_PRIVATE|ZEND_ACC_FINAL))) {
        fbc = NULL;
    }
}

"Why", you ask? The answer is: because that's how it works. In language design, this is called "name hiding", and it's up to the language to specify how name hiding works. Take C++ for example. It has well-defined, and complex name hiding rules. PHP has its own rules. They're different from C++. But they're unique to PHP. This is just something you have to memorize about the language.

I admit the docs could better spell this out, however.