File Edit

Hello, I want to be able to edit part of a file. via a text box using php how do i read a file and get the part i want to read and edit it and then write it back to that file here is an example: ..... case "$1" in start) 100.123.456.789 321.654.987.231 123.45.456.789 123.456.789.12 ;; ........ In this example I want to be able to be able to pull the ipaddresses out and edit them and add a new one on to the end and then write the data back to the file. Thanks in advance Randy