A quick-n-dirty way

If all you want to do is see if you are running PHP 5.3+, then just check for the existence of the array_replace() function, which was added in PHP 5.3:

<?php
if(function_exists('array_replace')) {
    // running PHP 5.3+
} else {
    // running something prior to PHP 5.3
}
?>

The “right way”

The version_compare() method can be used in conjunction with the PHP_VERSION constant to compare standardized PHP version strings. It also makes for more readable code:

if(version_compare(PHP_VERSION, '5.3.0') >= 0) {
    echo 'I am at least PHP version 5.3.0, my version: ' . PHP_VERSION . "\n";
}

Version constants

If you need to get down to the nitty gritty specifics of the PHP version you are running, use the PHP_VERSION_ID, PHP_MAJOR_VERSION, PHP_MINOR_VERSION, and PHP_RELEASE_VERSION constants, which were added in PHP 5.2.7. To ensure backward compatibility, the following code snippet from php.net will define these constants if they are undefined in your PHP version:

<?php
// PHP_VERSION_ID is available as of PHP 5.2.7, if our
// version is lower than that, then emulate it
if (!defined('PHP_VERSION_ID')) {
    $version = explode('.', PHP_VERSION);
    define('PHP_VERSION_ID', ($version[0] * 10000 + $version[1] * 100 + $version[2]));
}

// PHP_VERSION_ID is defined as a number, where the higher the number
// is, the newer a PHP version is used. It's defined as used in the above
// expression:
//
// $version_id = $major_version * 10000 + $minor_version * 100 + $release_version;
//
// Now with PHP_VERSION_ID we can check for features this PHP version
// may have, this doesn't require to use version_compare() everytime
// you check if the current PHP version may not support a feature.
//
// For example, we may here define the PHP_VERSION_* constants thats
// not available in versions prior to 5.2.7

if (PHP_VERSION_ID < 50207) {
    define('PHP_MAJOR_VERSION',   $version[0]);
    define('PHP_MINOR_VERSION',   $version[1]);
    define('PHP_RELEASE_VERSION', $version[2]);
    // and so on, ...
}
?>