Creating the Singleton design pattern in PHP5

Question

How would one create a Singleton class using PHP5 classes?

Answer 1

I know this is probably going to cause an unnecessary flame war, but I can see how you might want more than one database connection, so I would concede the point that singleton might not be the best solution for that... however, there are other uses of the singleton pattern that I find extremely useful.

Here's an example: I decided to roll my own MVC and templating engine because I wanted something really lightweight. However, the data that I want to display contains a lot of special math characters such as ≥ and μ and what have you... The data is stored as the actual UTF-8 character in my database rather than pre-HTML-encoded because my app can deliver other formats such as PDF and CSV in addition to HTML. The appropriate place to format for HTML is inside the template ("view" if you will) that is responsible for rendering that page section (snippet). I want to convert them to their appropriate HTML entities, but PHPs get_html_translation_table() function is not super fast. It makes better sense to retrieve the data one time and store as an array, making it available for all to use. Here's a sample I knocked together to test the speed. Presumably, this would work regardless of whether the other methods you use (after getting the instance) were static or not.

class EncodeHTMLEntities {

    private static $instance = null;//stores the instance of self
    private $r = null;//array of chars elligalbe for replacement

    private function __clone(){
    }//disable cloning, no reason to clone

    private function __construct()
    {
        $allEntities = get_html_translation_table(HTML_ENTITIES, ENT_NOQUOTES);
        $specialEntities = get_html_translation_table(HTML_SPECIALCHARS, ENT_NOQUOTES);
        $this->r = array_diff($allEntities, $specialEntities);
    }

    public static function replace($string)
    {
        if(!(self::$instance instanceof self) ){
            self::$instance = new self();
        }
        return strtr($string, self::$instance->r);
    }
}
//test one million encodings of a string
$start = microtime(true);
for($x=0; $x<1000000; $x++){
    $dump = EncodeHTMLEntities::replace("Reference method for diagnosis of CDAD, but clinical usefulness limited due to extended turnaround time (≥96 hrs)");
}
$end = microtime(true);
echo "Run time: ".($end-$start)." seconds using singleton\n";
//now repeat the same without using singleton
$start = microtime(true);
for($x=0; $x<1000000; $x++){
    $allEntities = get_html_translation_table(HTML_ENTITIES, ENT_NOQUOTES);
    $specialEntities = get_html_translation_table(HTML_SPECIALCHARS, ENT_NOQUOTES);
    $r = array_diff($allEntities, $specialEntities);
    $dump = strtr("Reference method for diagnosis of CDAD, but clinical usefulness limited due to extended turnaround time (≥96 hrs)", $r);
}
$end = microtime(true);
echo "Run time: ".($end-$start)." seconds without using singleton";

Basically, I saw typical results like this:

php test.php
Run time: 27.842966794968 seconds using singleton
Run time: 237.78191494942 seconds without using singleton

So while I'm certainly no expert, I don't see a more convenient and reliable way to reduce the overhead of slow calls for some kind of data, while making it super simple (single line of code to do what you need). Granted my example only has one useful method, and therefore is no better than a globally defined function, but as soon as you have two methods, you're going to want to group them together, right? Am I way off base?

Also, I prefer examples that actually DO something, since sometimes it's hard to visualise when an example includes statements like "//do something useful here" which I see all the time when searching for tutorials.

Anyway, I'd love any feedback or comments on why using a singleton for this type of thing is detrimental (or overly complicated).