SELECT MAX(id) FROM table

<?php
  $random_num = rand(0, $max(id));
?>

SELECT * FROM table WHERE id >= $random_num LIMIT 1

This code is useful if you have a table where some records have been deleted.  This code will favor records after large blocks of deleted records.

1) Get max id
2) generate random number with the max id
3) Query DB with random number.

Get random record in table

MySQL