TCPHP

I am trying to learn how to write user (form) submitted data to a flat text file, rather than an sql database, because I don't have access to one. I need to be able to check user input against records in this file. Right now I am using the POST method of form submitting and am using the "extract($_POST, EXTR_OVERWRITE)" to get the variables into my php handling script. Here is my problem: I am trying to use the fopen, fwrite and fclose functions to write user input initially into the text file I have (.txt) but it says it doesn't have permission to access the file. What's wrong? Also, if anyone could please direct me to a site that has moderatly extensive resources on flat-file databases, I would greatly appreciate it. ~Nathan