PHP不能在类方法中使用$ this作为默认参数

Question

Why cant I do this?

class Foo {

    private $val = array(
        'fruit' => 'apple',
        'color' => 'red'
    );

    function __construct( $arg = $this->val['color'] ) {

        echo $arg

    }

}

$bar = Foo;

I have also tried this:

class Foo {

    private static $val = array(
        'fruit' => 'apple',
        'color' => 'red'
    );

    function __construct( $arg = self::val['color'] ) {

        echo $arg

    }

}

$bar = Foo;

I need to be able to provide default values for some of my method arguments from variables already defined within the class.

Answer 1

You can try it like below;

class Foo {

private $val = array(
'fruit' => 'apple',
'color' => 'red'
);

function __construct($arg=null) {

echo ($arg==null) ? $this->val['color'] : $arg;

}

}

$bar = new Foo; // Output 'red'

This will echo your default color in $val array defined in class or you can pass the initial $arg value so it will override the default;

$bar = new Foo('Yellow'); // Output 'Yellow'

Answer 2

Constructor is called when object of that class created and you are trying to pass $this in constructor parameter default value so at that $this is not available to your constructor.

$this is available only after Constructorget called.

so please try this

class Foo {

    private $val = array(
        'fruit' => 'apple',
        'color' => 'red'
    );

    function __construct( $arg = NULL ) {
        echo $arg===NULL?$this->val['color'] : $arg;

    }

}

$bar = Foo;