How to don´t write if exist

Hi! I want php to check if a word already exist and then don´t write it to the file. This is my file list.txt: car, cat, dog, ... and my php which need some more function: ------ $fp = fopen("list.txt", "a"); $string = "$word".","." "; fwrite ($fp, $string); fclose ($fp); ------- I want to add: If the variable word exist, don´t add it to the string. Thanks for any tips. Regards Jan