Multiple constructors in PHP
Constructors are a really important part of the class-based object oriented programming. Through constuctors, a newly created objects initialize properties automatically before it is usable. In PHP, a constructor is a method named __construct(), which the keyword new automatically calls after creating the object. Constructors can also accept arguments, in which case, when the new statement is written, you also need to send the constructor arguments for the parameters.
You may also like: Method overloading in PHP
Here’s an example of how a constructor can be defined in PHP.
<?php
class Person
{
private string $name;
public function __construct(string $name)
{
$this->name = $name;
}
public function getName() : string
{
return $this->name;
}
}
Now, whenever an object of class Person gets created like so.
$person = new Person('Foo');
…constructor gets called, and it assigns “Foo” to $name which then can be accessed by calling getName() method.
Now, what if you want to introduce an another constructor into this class which have different sets of arguments. For instance, you want to intialize $name and $age of the person with this constructor, how would you do that?
Well, the simple answer is, You can’t. At least natively. PHP lacks support for declaring multiple constructors of different numbers of parameters for a class unlike languages such as Java. So, if we declare an another constructor in the above example like so.
<?php
class Person
{
private string $name;
private string $age;
public function __construct(string $name)
{
$this->name = $name;
}
public function __construct(string $name, string $age)
{
$this->name = $name;
$this->age = $age;
}
public function getName() : string
{
return $this->name;
}
}
$person1 = new Person('Amit');
$person2 = new Person('Amit', '12');
PHP will throw a fatal error upon running the above code.
<b>Fatal error</b>: Cannot redeclare Person::__construct() in <b>[...][...]</b> on line <b>13</b><br />
There’s a workaround though through which one can leverage the use of multiple constructors in a class.
The workaround
Here’s a little snippet which demonstrate how to utilize multiple constructors in PHP.
<?php
class Animal
{
public function __construct()
{
$arguments = func_get_args();
$numberOfArguments = func_num_args();
if (method_exists($this, $function = '__construct'.$numberOfArguments)) {
call_user_func_array(array($this, $function), $arguments);
}
}
public function __construct1($a1)
{
echo('__construct with 1 param called: '.$a1.PHP_EOL);
}
public function __construct2($a1, $a2)
{
echo('__construct with 2 params called: '.$a1.','.$a2.PHP_EOL);
}
public function __construct3($a1, $a2, $a3)
{
echo('__construct with 3 params called: '.$a1.','.$a2.','.$a3.PHP_EOL);
}
}
$o = new Animal('sheep');
$o = new Animal('sheep','cat');
$o = new Animal('sheep','cat','dog');
// __construct with 1 param called: sheep
// __construct with 2 params called: sheep,cat
// __construct with 3 params called: sheep,cat,dog
As you can see, the class has a standard constructor and rest of the three are normal class methods. But the magic lies in the constructor itself.
Here, we first need to count the number of arguments passed to the constructor using func_num_args(), and get all the assigned arguments by func_get_args(). Then all that’s left for us to do is to call the suitable function using call_user_func_array() by passing the original arguments as call_user_func_array()’s second argument. And voila! you’ve written yourself a functional multiple constructor implementation in PHP.
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