help in php and mysql query and function

i have a table in mysql as an example --id---name--number-- --2-----joe------5----- --3-----dan-----7----- --4-----jake-----9----- --4-----bill------10----- --4-----mike----11----- --5-----chris----22---- i have a variable such as $ext_number the variable value can contain different number every time i need a query that will check the variable number against the "number" column in the database. lets say that the $ext_number is equal to 9, the query needs to check the variable against the number 9 (not below) at the database rows, and continue to count up by 1 increments until it will reach a number that is not used. once it will reach the number that is not used i should get this number under a new variable called $ext_number2