PHP CookBook

Recipe 2.6 Generating Random Numbers Within a Range

2.6.1 Problem

You want to generate a random number within a range of numbers.

2.6.2 Solution

Use mt_rand( ):

// random number between $upper and $lower, inclusive
$random_number = mt_rand($lower, $upper);

2.6.3 Discussion

Generating random numbers is useful when you want to display a random image on a page, randomize the starting position of a game, select a random record from a database, or generate a unique session identifier.

To generate a random number between two end points, pass mt_rand( ) two arguments:

$random_number = mt_rand(1, 100);

Calling mt_rand( ) without any arguments returns a number between 0 and the maximum random number, which is returned by mt_getrandmax( ).

Generating truly random numbers is hard for computers to do. Computers excel at following instructions methodically; they're not so good at spontaneity. If you want to instruct a computer to return random numbers, you need to give it a specific set of repeatable commands; the very fact that they're repeatable undermines the desired randomness.

PHP has two different random number generators, a classic function called rand( ) and a better function called mt_rand( ). MT stands for Mersenne Twister, which is named for the French monk and mathematician Marin Mersenne and the type of prime numbers he's associated with. The algorithm is based on these prime numbers. Since mt_rand( ) is more random and faster than rand( ), we prefer it to rand( ).

If you're running a version of PHP earlier than 4.2, before using mt_rand( ) (or rand( )) for the first time in a script, you need to seed the generator, by calling mt_srand( ) (or srand( )). The seed is a number the random function uses as the basis for generating the random numbers it returns; it's how to solve the repeatable versus random dilemma mentioned earlier. Use the value returned by microtime( ) , a high-precision time function, to get a seed that changes very quickly and is unlikely to repeat — qualities desirable in a good seed. After the initial seed, you don't need to reseed the randomizer. PHP 4.2 and later automatically handles seeding for you, but if you manually provide a seed before calling mt_rand( ) for the first time, PHP doesn't alter it by substituting a new seed of its own.

If you want to select a random record from a database — an easy way is to find the total number of fields inside the table — select a random number in that range, and then request that row from the database:

$sth = $dbh->query('SELECT COUNT(*) AS count FROM quotes');
if ($row = $sth->fetchRow()) {
    $count = $row[0];
} else {
    die ($row->getMessage());
}

$random = mt_rand(0, $count - 1);

$sth = $dbh->query("SELECT quote FROM quotes LIMIT $random,1");
while ($row = $sth->fetchRow()) {
    print $row[0] . "\n";
}

This snippet finds the total number of rows in the table, computes a random number inside that range, and then uses LIMIT $random,1 to SELECT one line from the table starting at position $random.

Alternatively, if you're using MySQL 3.23 or above, you can do this:

$sth = $dbh->query('SELECT quote FROM quotes ORDER BY RAND() LIMIT 1');
while ($row = $sth->fetchRow()) {
    print $row[0] . "\n";
}

In this case, MySQL randomizes the lines, and then the first row is returned.

2.6.4 See Also

Recipe 2.7 for how to generate biased random numbers; documentation on mt_rand( ) at http://www.php.net/mt-rand and rand( ) at http://www.php.net/rand; the MySQL Manual on RAND( ) at http://www.mysql.com/doc/M/a/Mathematical_functions.html.