Question

How can i perform a function once a variable’s value has been set?

say like

$obj = new object(); // dont perform $obj->my_function() just yet
$obj->my_var = 67    // $obj->my_function() now gets run

I want the object to do this function and now having to be called by the script.

Thanks

EDIT
my_var is predefined in the class, __set is not working for me.

Answer 1

Use a private property so __set() is invoked:

class Myclass {
  private $my_var;
  private $my_var_set = false;

  public function __set($var, $value) {
    if ($var == 'my_var' && !$this->my_var_set) {
      // call some function
      $this->my_var_set = true;
    }
    $this->$var = $value;
  }

  public function __get($var, $value) {
    return $this->$name;
  }
}

See Overloading. __set() is called because $my_var is inaccessible and there is your hook.