PHP OOP Singleton Class

Question

I have the code

public static function constructMe() {
    if(!$this->_instance instanceof self) {
    $this->_instance = new self();
    }
    return $this->_instance;
}

To instansiate the class, and there is a $_instance variable in the class, but I'm getting the error:

Fatal error: Using $this when not in object context

Halcyon · Accepted Answer · 2011-06-10 21:21:59Z

7

You need to use

self::$_instance

Since you're in a static scope (you have no $this)

Also, make sure you declare

private static $_instance;

Also, I don't know if new self(); works You could try new __CLASS__() or just write the name of the class ..

And don't use an instanceof, just check with isset or empty (it's more accurate)

And be careful using !$var instanceof something, always write as !($var instanceof something) because you don't want to accidentally cast to a boolean.

answered Jun 10, 2011 at 21:21

Halcyon

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14 Comments

alexn Over a year ago

Yes you can do new self();.

Joshwaa Over a year ago

Yeah, but if it's equal to "cake" or something, then you know.. It just seems safer to use instanceof.

Halcyon Over a year ago

If it's being set to "cake" you have bigger problems ;)

Matthew Over a year ago

if (!self::$instance) self::$instance = new self(); sufficiently solves the problem.

David Harkness Over a year ago

@Frits - Once you've already thrown in the towel and decided to use the singleton pattern, all bets are off. :) However, if (!self::$_instance) is perfectly acceptable. Since $_instance is private (it better be), it can only be null or an instance of the class. If you write code that allows it to be anything other, only then would I support a career change.

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