problem in file handling help me..........

how to append text at a specific position in a file in php

for example I'm created a file "temp.txt"
and in this file some content are write
like this.........

"hi my name is prince. I'm working in ncr zone."

my prob is...
I want replace this line "I'm working in ncr zone." Here i want "I'm interested in web development"

Plz help me..............

 

then read the file using fread, use str_replace to replace the text and again write/append in the file.