I screwed around with it -- commented out your for loop, I do not believe it is necessary...I have NOT tested this, but maybe you could try something along these lines. You also should call the fclose() function, I added that.
<?PHP
$i = "1";
$signersfile = fopen("$username.comments","r");
while (!feof ($signersfile)):
$signers = fgets($signersfile,4096);
$split = split( "_", $signers);
//for($i = 0;$i <= $split[0];$i++){
echo "<P><INPUT type=\"checkbox\" name=\"del$i++\" value=\"$split[0]\">Delete$split[0]\n";
//}
endwhile;
fclose($signersfile);
?>
--ph