file_put_contents, file_append and line breaks

Did you tried with PHP EOL constant?

file_put_contents($filename, $commentnumber . PHP_EOL, FILE_APPEND)

--- Added ---

I just realize that my file editor does the same, but don't worrie, is just a ghost character that the editor places there to signal that there is a newline You could try this

A file with EOL after the last number looks like:
1_
2_
3_
EOF

but a file without that last character looks like

1_
2_
3
EOF

where _ means a space character

You could try to parse the file contents using php to see what's inside

$lines = explode( PHP_EOL, file_get_contents($file));
foreach($lines as $line ) {
    var_dump($line);
}

...tricky

pauls answer has the correct approach but he has a mistake. what you need ist the following:

file_put_contents($filename, trim($commentnumber).PHP_EOL, FILE_APPEND);

the PHP_EOL constant makes sure to use the right line ending on mac, windows and unix systems the trim function removes any newline or whitespace on both sides of the string. converting to integer would be a huge mistake because 1. you might end up having zero, expecially because of white space or special characters (wherever they come from...) 2. ids dont necessarily need to be integers

annoyingregistration, what you have there is absolutely fine. PHP_EOL and "\n" are exactly the same.

The code you provided theres nothing wrong with it so it must be the value of $commentnumber that has a space at the end of it. as stated, run your $commentnumber through the trim() function.

file_put_contents($filename, trim($commentnumber . "\n"), FILE_APPEND);

Good luck.