Why am I getting Fatal error when calling a parent's constructor?

Question

I am extending one of the SPL (Standard PHP Library) classes and I am unable to call the parent\'s constructor. Here is the error I am getting:

Fatal erro

Answer 1

If you want to call the constructor of the nearest ancestor, you can loop through the ancestors with class_parents and check with method_exists if it has a constructor. If so, call the constructor; if not, continue your search with the next nearest ancestor. Not only do you prevent overriding the parent's constructor, but also that of other ancestors (in case the parent doesn't have a constructor):

class Queue extends SplQueue {

  public function __construct() {
    echo 'before';

    // loops through all ancestors
    foreach(class_parents($this) as $ancestor) {

      // check if constructor has been defined
      if(method_exists($ancestor, "__construct")) {

        // execute constructor of ancestor
        eval($ancestor."::__construct();");

        // exit loop if constructor is defined
        // this avoids calling the same constructor twice
        // e.g. when the parent's constructor already
        // calls the grandparent's constructor
        break;
      }
    }
    echo 'I have made it after the parent constructor call';
  }

}

For code reuse, you could also write this code as a function that returns the PHP code to be evaled:

// define function to be used within various classes
function get_parent_construct($obj) {

  // loop through all ancestors
  foreach(class_parents($obj) as $ancestor) {

    // check if constructor has been defined
    if(method_exists($ancestor, "__construct")) {

      // return PHP code (call of ancestor's constructor)
      // this will automatically break the loop
      return $ancestor."::__construct();";
    }
  }
}

class Queue extends SplQueue {

  public function __construct() {
    echo 'before';

    // execute the string returned by the function
    // eval doesn't throw errors if nothing is returned
    eval(get_parent_construct($this));
    echo 'I have made it after the parent constructor call';
  }
}

// another class to show code reuse
class AnotherChildClass extends AnotherParentClass {

  public function __construct() {
    eval(get_parent_construct($this));
  }
}