Make checks for submit buttons - php

Question

I encountered some problems, I want this script to:
Open test.txt file.
Check if user have added any text to the txt file.
If user have added any text, delete the existing line and replace it with the new. From $_POST.
If user have not, add $_POST in test.txt
Problem:
When I spam the submit button, the .txt will mess up. Anyone know how to make checks, so it does not mess up?
Please don't suggest MYSQL, I need these in .txt file.
Thanks.
function cutline($filename,$line_no=-1) {
$strip_return=FALSE;
$data=file($filename);
$pipe=fopen($filename,'w');
$size=count($data);
if($line_no==-1) $skip=$size-1;
else $skip=$line_no-1;
for($line=0;$line<$size;$line++)
if($line!=$skip)
fputs($pipe,$data[$line]);
else
$strip_return=TRUE;
return $strip_return;
}
if ($userid = 1) {
if(!isset($_POST['submit'])){
?>
<center><form action="" method="POST">
<b>HWID</b>
<input type="text" name="HWID" />
<input type="submit" value="Add HWID" name="submit">
</form>
</center>
<?php
}else{
$userid= 1;
$userid = "user=" . $userid;
$file = "test.txt";
$lines = file($file);
$count = 1;
foreach ($lines as $e) {
if(strpos($e, $userid) !== FALSE){
cutline($file,$count);
++$count;
}
}
$fh = fopen($file, 'a') or die("can't open file");
$stringData = $userid . $_POST['HWID'] . "\n";
fwrite($fh, $stringData);
}
}
}else{
echo "You're not logged in";
}
?>

Answer 1

I am not 100% sure how the text file is messing up, but I guess locking won't help here as locks are released when the script finished (or is reloaded).
It looks like you just "kill" your cutline while in progress and the remaining lines will not be written. One way to fix this could be to save the new content of the file in a temporary variable and call fwrite only once. (I am not 100% sure if this will work)
Another possibility is to write the results of cutline into a temporary file and replace the old file with the new one, when the cutline method is done. This can happen inside the method.
In either ways the existing file will not be touched if the script gets killed in an unsafe state. But you can still loose the new input from the user when he manages to reload the page right after the function call of cutline and before you add the new input in this line
fwrite($fh, $stringData);
I think this is really hard to force as this operation is quite fast.
EDIT:
Don't forget to test the script using multiple users at the same time, if this is a valid use case. If two or more guys are editing the same file at the same time it will mess up as well. So you might end up with some locking but that will not solve the problem described here.