Generators In PHP

PHP 5.5 added a really nice feature: Generators.

Generators allow you to create a function that returns an list of items without having to create a (possibly very large) array first.

Here’s a simple “classic” generator example. Suppose you wanted to create a range function (PHP provides this but humour me :))

Our naive implementation will simply take a start value, an end value and return the range as an array:

function myRange($start, $end){
    $results = array();
    for($x = $start; $x < $end; $x++){
      $results[] = $x;
    }
    return $results;
}

This will work fine, however if you attempt to loop over a very large number

$num = 10000000;
foreach(myRange(1,$num) as $item){
  echo $item."\n";
}

You’ll that the script hangs for a very long time and may end up dying with a memory error.

This is because the myRange function will attempt to create an array with 10,000,000 entries and loop over that.

A generator will greatly decrease the amount of memory needed. To implement the generator we simply yield every value we want to return:

function generatorRange($start, $end){
    for($x = $start; $x < $end; $x++){
      yield $x;
    }
}

Now if we attempt to loop over using a very large number we find that, not only do we start getting results immediately but the memory usage is much lower.

$num = 10000000;
foreach(generatorRange(1,$num) as $item){
  echo $item."\n";
}

This is because PHP only has to store the information about the current state of the generator rather than storing all 10,000,000 numbers.

Have a cool use for generators? Let me know..