php如何调用父类构造方法？

 title = $title; // 给属性 title 赋传进来的值 $this -> producerFirstName= $firstName; $this -> producerMainName = $mainName; $this -> price= $price; } function getProducer(){ // 声明方法 return "{$this -> producerFirstName }"."{$this -> producerMainName}"; } function getSummaryLine(){ $base = "{$this->title}( {$this->producerMainName},"; $base .= "{$this->producerFirstName} )"; return $base; } } class CdProduct extends ShopProduct { public $playLenth; function __construct($title,$firstName,$mainName,$price,$playLenth){ parent::__construct($title,$firstName,$mainName,$price); $this -> playLenth= $playLenth; } function getPlayLength(){ return $this -> playLength; } function getSummaryLine(){ $base = "{$this->title}( {$this->producerMainName},"; $base .= "{$this->producerFirstName} )"; $base .= "：playing time - {$this->playLength} )"; return $base; } } // 定义类 class BookProduct extends ShopProduct { public $numPages; function __construct($title,$firstName,$mainName,$price,$numPages){ parent::__construct($title,$firstName,$mainName,$price); $this -> numPages= $numPages; } function getNumberOfPages(){ return $this -> numPages; } function getSummaryLine(){ $base = "{$this->title}( {$this->producerMainName},"; $base .= "{$this->producerFirstName} )"; $base .= "：page cont - {$this->numPages} )"; return $base; } } ?>