{"id":643,"date":"2021-03-24T07:37:53","date_gmt":"2021-03-24T07:37:53","guid":{"rendered":"https:\/\/phptutorial.net\/?page_id=643"},"modified":"2025-04-07T11:23:41","modified_gmt":"2025-04-07T11:23:41","slug":"php-call-parent-constructor","status":"publish","type":"page","link":"https:\/\/www.phptutorial.net\/php-oop\/php-call-parent-constructor\/","title":{"rendered":"How to Call the Parent Constructor"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn how to call the parent constructor from the constructor of the child class.<\/p>\n\n\n\n

Child class doesn’t have a constructor #<\/a><\/h2>\n\n\n\n

In the inheritance tutorial<\/a>, you have learned how to define the SavingAccount<\/code> class that inherits the BankAccount<\/code> class:<\/p>\n\n\n

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\"php<\/figure>\n<\/div>\n\n\n

However, we haven’t discussed the constructors<\/a> of the parent and child classes in the context of inheritance.<\/p>\n\n\n\n

The following adds a constructor to the BankAccount<\/code> class, which accepts the $balance<\/code> parameter. The constructor assigns the $balance<\/code> argument to the $balance<\/code> property:<\/p>\n\n\n

<?php<\/span>\n\nclass<\/span> BankAccount<\/span>\n<\/span>{\n\tprivate<\/span> $balance;\n\n\tpublic<\/span> function<\/span> __construct<\/span>($balance)<\/span>\n\t<\/span>{\n\t\t$this<\/span>->balance = $balance;\n\t}\n\n\tpublic<\/span> function<\/span> getBalance<\/span>()<\/span>\n\t<\/span>{\n\t\treturn<\/span> $this<\/span>->balance;\n\t}\n\n\tpublic<\/span> function<\/span> deposit<\/span>($amount)<\/span>\n\t<\/span>{\n\t\tif<\/span> ($amount > 0<\/span>) {\n\t\t\t$this<\/span>->balance += $amount;\n\t\t}\n\n\t\treturn<\/span> $this<\/span>;\n\t}\n}<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
The SavingAccount<\/code> class remains the same and doesn’t include its own constructor:<\/p>\n\n\n
<?php<\/span> \n\nclass<\/span> SavingAccount<\/span> extends<\/span> BankAccount<\/span>\n<\/span>{\n\tprivate<\/span> $interestRate;\n\n\tpublic<\/span> function<\/span> setInterestRate<\/span>($interestRate)<\/span>\n\t<\/span>{\n\t\t$this<\/span>->interestRate = $interestRate;\n\t}\n\n\tpublic<\/span> function<\/span> addInterest<\/span>()<\/span>\n\t<\/span>{\n\t\t\/\/ calculate interest<\/span>\n\t\t$interest = $this<\/span>->interestRate * $this<\/span>->getBalance();\n\t\t\/\/ deposit interest to the balance<\/span>\n\t\t$this<\/span>->deposit($interest);\n\t}\n}<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
When you create a new instance of the SavingAccount<\/code>, PHP will call the constructor of the SavingAccount<\/code> class. However, PHP cannot find the constructor in the SavingAccount class<\/code>. Therefore, it continues to search for the constructor of the parent class of the SavingAccount<\/code> class, which is the BankAccount<\/code> class. And it invokes the constructor of the BankAccount<\/code> class.<\/p>\n\n\n\n
If you don’t pass an argument to the constructor of the SavingAccount<\/code> class, you’ll get an error:<\/p>\n\n\n
$account = new<\/span> SavingAccount();<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Error:<\/p>\n\n\n
Fatal error: Uncaught ArgumentCountError: Too few arguments to function<\/span> BankAccount<\/span>::__construct<\/span>()<\/span>, 0 passed<\/span> in<\/span> ... on<\/span> line<\/span> 5 and<\/span> exactly<\/span> 1 expected<\/span> in<\/span> ...<\/span><\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
But if you pass an argument to the constructor, it’ll work perfectly:<\/p>\n\n\n
$account = new<\/span> SavingAccount(100<\/span>);<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Define a constructor in the child class #<\/a><\/h2>\n\n\n\n
A child class can have its own constructor. For example, you can add a constructor to the SavingAccount<\/code> class as follows:<\/p>\n\n\n
<?php<\/span>\n\nclass<\/span> SavingAccount<\/span> extends<\/span> BankAccount<\/span>\n<\/span>{\n\tprivate<\/span> $interestRate;\n\n\tpublic<\/span> function<\/span> __construct<\/span>($interestRate)<\/span>\n\t<\/span>{\n\t\t$this<\/span>->interestRate = $interestRate;\n\t}\n\n\tpublic<\/span> function<\/span> setInterestRate<\/span>($interestRate)<\/span>\n\t<\/span>{\n\t\t$this<\/span>->interestRate = $interestRate;\n\t}\n\n\tpublic<\/span> function<\/span> addInterest<\/span>()<\/span>\n\t<\/span>{\n\t\t\/\/ calculate interest<\/span>\n\t\t$interest = $this<\/span>->interestRate * $this<\/span>->getBalance();\n\t\t\/\/ deposit interest to the balance<\/span>\n\t\t$this<\/span>->deposit($interest);\n\t}\n}<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
The SavingAccount<\/code> class has a constructor that initializes the interestRate<\/code> property.<\/p>\n\n\n\n
When a child class has its own constructor, the constructor in the child class will not call the constructor of its parent class automatically. <\/p>\n\n\n\n
For example, the following creates a new instance of the SavingAccount<\/code> class and initializes the $interestRate<\/code> property to the value 0.05<\/code> <\/p>\n\n\n
$account = new<\/span> SavingAccount(0.05<\/span>);<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
To call the constructor of the parent class from the constructor of the child class, you use the parent::__construct(arguments)<\/code> syntax. <\/p>\n\n\n\n
The following changes the constructor of the SavingAccount<\/code> class that accepts two arguments: balance & interest rate. It also calls the constructor of the BankAccount<\/code> class to initialize the $balance<\/code> property:<\/p>\n\n\n
<?php<\/span>\n\n\nclass<\/span> SavingAccount<\/span> extends<\/span> BankAccount<\/span>\n<\/span>{\n\tprivate<\/span> $interestRate;\n\n\tpublic<\/span> function<\/span> __construct<\/span>($balance, $interestRate)<\/span>\n\t<\/span>{\n\t\tparent<\/span>::__construct($balance);\n\n\t\t$this<\/span>->interestRate = $interestRate;\n\t}\n\n\tpublic<\/span> function<\/span> setInterestRate<\/span>($interestRate)<\/span>\n\t<\/span>{\n\t\t$this<\/span>->interestRate = $interestRate;\n\t}\n\n\tpublic<\/span> function<\/span> addInterest<\/span>()<\/span>\n\t<\/span>{\n\t\t\/\/ calculate interest<\/span>\n\t\t$interest = $this<\/span>->interestRate * $this<\/span>->getBalance();\n\t\t\/\/ deposit interest to the balance<\/span>\n\t\t$this<\/span>->deposit($interest);\n\t}\n}<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
The syntax for calling the parent constructor is the same as a regular method.<\/p>\n\n\n\n
Put it all together:<\/p>\n\n\n
<?php<\/span>\n\nclass<\/span> BankAccount<\/span>\n<\/span>{\n\tprivate<\/span> $balance;\n\n\tpublic<\/span> function<\/span> __construct<\/span>($balance)<\/span>\n\t<\/span>{\n\t\t$this<\/span>->balance = $balance;\n\t}\n\n\tpublic<\/span> function<\/span> getBalance<\/span>()<\/span>\n\t<\/span>{\n\t\treturn<\/span> $this<\/span>->balance;\n\t}\n\n\tpublic<\/span> function<\/span> deposit<\/span>($amount)<\/span>\n\t<\/span>{\n\t\tif<\/span> ($amount > 0<\/span>) {\n\t\t\t$this<\/span>->balance += $amount;\n\t\t}\n\n\t\treturn<\/span> $this<\/span>;\n\t}\n}\n\nclass<\/span> SavingAccount<\/span> extends<\/span> BankAccount<\/span>\n<\/span>{\n\tprivate<\/span> $interestRate;\n\n\tpublic<\/span> function<\/span> __construct<\/span>($balance, $interestRate)<\/span>\n\t<\/span>{\n\t\tparent<\/span>::__construct($balance);\n\t\t$this<\/span>->interestRate = $interestRate;\n\t}\n\n\tpublic<\/span> function<\/span> setInterestRate<\/span>($interestRate)<\/span>\n\t<\/span>{\n\t\t$this<\/span>->interestRate = $interestRate;\n\t}\n\n\tpublic<\/span> function<\/span> addInterest<\/span>()<\/span>\n\t<\/span>{\n\t\t$interest = $this<\/span>->interestRate * $this<\/span>->getBalance();\n\t\t$this<\/span>->deposit($interest);\n\t}\n}\n\n$account = new<\/span> SavingAccount(100<\/span>, 0.05<\/span>);\n$account->addInterest();\necho<\/span> $account->getBalance();<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
The following class diagram illustrates inheritance between the SavingAccount<\/code> and BankAccount<\/code> classes:<\/p>\n\n\n
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\"\"<\/figure>\n<\/div>\n\n\n
Summary #<\/a><\/h2>\n\n\n\n
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  • The constructor of the child class doesn’t automatically call the constructor of its parent class.<\/li>\n\n\n\n
  • Use parent::__construct(arguments)<\/code> to call the parent constructor from the constructor in the child class.<\/li>\n<\/ul>\n
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    Did you find this tutorial useful?<\/div>\n\t\t
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Try it<\/a><\/p>\n\n\n\n