strpos problem (offset not contained in string)

I am not sure what the problem is, I am getting the error: "strpos() [function.strpos]: Offset not contained in string." on lines
97 , 101, 126, and 129

The link to the source is :
http://phpfi.com/250450

That error message is saying "The value of $start is greater than the length of $pagecontents".

Add an "if" statement to bypass the function if $start is greater than the length of $pagecontents.

What part do I add the ifstatment to, and what will the if statement say?