How do I return the key of a certain value in an array?

http://php.net/manual/en/function.array-search.php

In this case, you could use

 $key = array_search('25 Results',$options)

to find the key that matches the value.

One easy way is to use array_flip:

$options = array(
                    '10' => '10 Results',
                    '15' => '15 Results',
                    '20' => '20 Results',
                    '25' => '25 Results',
                    '30' => '30 Results'
                );
$reverseoptions = array_flip($options);

Then just do $reverseoptions['30 Results']; //returns 30;

Limitations exist. You can only do this with a simple array; it cannot be recursive without doing a little more code to make that happen. Also, if any values are alike, the later one will replace the first.

$test = array('1'=>'apple', '2'=>'pear','3'=>'apple');
$testflip = array_flip($test);
print_r($testflip);

//Outputs Array ( [apple] => 3 [pear] => 2 )

I do this often to convert database representations to readable strings.

An alternative to array_search is to use a foreach loop! This is in case you do not know what the key is beforehand.

foreach ($arr as $key => $value) {
    echo "Key: $key; Value: $value<br />\n";
}

You can access the keys of the array and do as you wish with them. This will come in handy for doing database conversions as you mentioned.