Wait till file contents changed

Hi, I am trying to figure out a way to do the following: I have a cli script that has to wait untill the contents of a file has changed from a "0" to a "1". If it is a "0", the script must wait, if it is changed to a "1" the script must continue. The "0" and "1" is written by a BASH script. When the bash script is run , it echo's a "0" into the file to tell php that the process has started and php should wait untill the process is finished which occurs when the BASH script echo's a "1" into the same file. Any pointers? Thank you Danny