only one at atime

Hi everyone, I create a php script that will run every minute (by cronjob) and update some database tables (php 5, database mysql 5, tables type innodb) the problem is that I want this script to run only one at atime (only one process from this script can run ) for example, if the cronjob start the script and the script takes more than one minute, then after this one minute the cronjob start another process from this script, it should exit once it find there is an old process of it running) what I do now is that when the script start it will check for a tmp file (/tmp/script.pid) and if it fine it it will exit. if the file (/tmp/script.pid) is not exists, it will create it and start doing the database update and when the script finish it will remove the file any better idea ? Thanks -- Ahmad http://www.v-tadawul.com