Help Counting fwrite. strpos()

[maerd]

What I want to do here is have it count from that string not anything before that string and write the line number to the txt file starting from that strpos()

$startingpoint="<pre>";
$startcount=strpos("$startingpoint");

$html=file("file.html");
$lines=count($html);

$writedata=$test.":".$lines."\n";
$file=fopen("file.html","a");
fwrite($file,$writedata);
fclose($file);