Call parent constructor in PHP

/*from w  w  w .ja v  a  2 s.co  m*/
<?php

class Calculator {
  protected $_val1, $_val2;

  public function __construct( $val1, $val2 ) {
    $this->_val1 = $val1;
    $this->_val2 = $val2;
  }

  public function add() {
    return $this->_val1 + $this->_val2;
  }

  public function subtract() {
    return $this->_val1 - $this->_val2;
  }

  public function multiply() {
    return $this->_val1 * $this->_val2;
  }

  public function divide() {
    return $this->_val1 / $this->_val2;
  }
}

class CalcAdvanced extends Calculator {
  private static $_allowedFunctions = array( "pow" => 2, "sqrt" => 1, "exp" => 1 );

  public function __construct( $val1, $val2=null ) {
    parent::__construct( $val1, $val2 );
  }

  public function __call( $methodName, $arguments ) {
    if ( in_array( $methodName, array_keys( CalcAdvanced::$_allowedFunctions ) ) ) {
      $functionArguments = array( $this->_val1 );
      if ( CalcAdvanced::$_allowedFunctions[$methodName] == 2 ) array_push( $functionArguments, $this->_val2 );
      return call_user_func_array( $methodName, $functionArguments );
    } else {
      die ( "<p>Method 'CalcAdvanced::$methodName' doesn't exist</p>" );
    }
  }
}
$ca = new CalcAdvanced( 3, 4 );
echo "<p>3 + 4 = " . $ca->add() . "</p>";
echo "<p>3 - 4 = " . $ca->subtract() . "</p>";
echo "<p>3 * 4 = " . $ca->multiply() . "</p>";
echo "<p>3 / 4 = " . $ca->divide() . "</p>";
echo "<p>pow( 3, 4 ) = " . $ca->pow() . "</p>";
echo "<p>sqrt( 3 ) = " . $ca->sqrt() . "</p>";
echo "<p>exp( 3 ) = " . $ca->exp() . "</p>";

?>