Deleting code from a text file

I have a form that updates a text file (ads.txt) with an image and url, to create a dynamic banner advertising page. The form references a script which is shown below.

I can add items to the text file, but don't know how to DELETE them. Please could someone show me how to amend the script in order for it to delete a line from the text file, rather than add it.

Thank you.

---------------------------------------------------------
THE CODE
---------------------------------------------------------

<?php

$adsfile = "ads.txt"; //the ads file

$page = $adsfile;
$filename = $adsfile;
$fd = fopen( $filename, "r" );
$current = fread( $fd, filesize( $filename ) );
fclose( $fd );

$ad = "<a href='$url' target='new'><IMG SRC='$adurl' border=0></a>";
$ad .= "<BR><BR>";
$ad .= "
";

if (file_exists("$page")){
$cartFile = fopen("$page","a[b]");
fputs($cartFile,$ad);
fclose($cartFile);
}
else
{
$cartFile = fopen("$page","a[b]");
fputs($cartFile,$ad);
fclose($cartFile);
}

php?>

<?php

//done

php?>

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one simple solution is to use str_replace to find any instances of a line and replace it with nothing, or you could use regular expressions to do the same thing.

however it would be better to use a database then a text file