`return $ this-> foo（）=== null'是什么意思？

Question

this is a code sample:

class md {
public function __construct() {
    if($this->getIsGuest()){
        echo 'iam guest';
    }
}
public function getIsGuest() {
    return $this->getState('__id')===null;
}
public function getState($val) {
    return '3';
}

is return $this->foo() === null is the default value of foo of nothing returned ?

Answer 1

getIsGuest() is returning the comparison of $this->getState('__id') to null.

If $this->getState('__id') returns exactly null , then getIsGuest() function returns true. If it returns anything except exactly null , then the getIsGuest() function returns false.

Answer 2

=== is type equality in php. In this case it would be used to tell the differencee between 0, '', false or the actual value null.

Answer 3

This isn't all too good a sample, but there's nothing default-value-ish to it. What you can be sure of, is that the getIsGuest method will always return a boolean (either true or false ). In your code snippet, it'll always return false, because it uses the return value of another method ( getState ) and compares it to null strictly (value and type). Since the getState member function is hard-coded to return a char in your example, it will never return null, and therefore, getIsGuest will always return false.

PHP has null as its default return value for all functions or methods that have no explicit return statement (with the possible exception of a constructor, which returns an instance of an object). Just like JS will return undefined by default (except constructors, again), or C functions can return void , but that's another matter.
In short: no, your code isn't setting a default return value. I'll attempt to clarify all this by editing your snippet a bit:

class md
{
    private $_mdData = array();//no data
    public function __construct(array $params = null)//default value of $params is null
    {//the constructor expects either no parameters, or an array
        if ($params !== null)
        {
            $this->_mdData = $params;
        }
        if($this->getIsGuest())
        {
            echo 'I\'m guest';
        }
    }

    public function getIsGuest()
    {
        return $this->getState('__id') === null;
    }

    public function getState($val)
    {//return whatever value is stored in the private array under $val-key
     //if the key doesn't exist, null is returned
        return (isset($this->_mdData[$val]) ? $this->_mdData[$val] : null);
    }
}

$guest = new md();//no params

constructor calls getIsGuest, which tries to access $ mdData[' _id']
$_mdData is empty, so the key doesn't exist and null is returned to getIsGuest.
getIsGuest compares the return value (null) to null, and returns true(because they are the same)
The constructor receives true, because that's where the getIsGuest method was called, and evaluates the value if ($this->getIsGuest()) --> true, so I'm a guest is echoed.

$nonGuest = new md(array('__id'=>123));//same drill, only this time getState will return 123, and getIsGuest returns false

Now the difference between == and === :

$nullish = new md(array('__id' => ''));

This won't echo I'm a guest, Unless you were to change return $this->getState('__id') === null; to return $this->getState('__id') == null; because '' an empty string is null-ish, like 0 is falsy etc...