Locking mysql tables with PHP

Hi bit confused! here's what I want to do: get a numeric value from a MySQL table, do a calculation, then on another PHPpage update the numeric value in the table. what I don't want is anyone else getting the same number from the table before I've updated it. what PHP would you use to do this? Sorry for what is probably a really basic question - I did look at the MySQL manual, but I'm not sure how exactly to issue an SQL query LOCK TABLES using PHP. TIA Tony