Help seeking in a file a specific string

Hi!

I want to read a file, and write something when a specific string is found.

For instance I have this file:

param1=32
param2=2342
param3=
param4=23423
param5=dfad

I want to write after 'param3'. I tried with this but it doesn't work.

$fp = fopen($file,'r+');
while(!feof($fp)){
	$buffer = fgets($fp,4096);
        if ($buffer == 'param3=')
        fwrite ($fp, '2323');
}

$buffer seems not to read 'param3'

Thanks in advance!
.