If you want to empty a file of it's contents bare in mind that opening a file in w mode truncates the file automatically, so instead of doing...

<?php
$fp = fopen("/tmp/file.txt", "r+");
ftruncate($fp, 0);
fclose($fp);
?>

You can just do...

<?php
$fp = fopen("/tmp/file.txt", "w");
fclose($fp);
?>

You MUST use rewind() after ftruncate() to replace file content

Writing after ftruncate

I didnt expect that I can write in the middle of nowhere. I thought that I would write at the beginning of the file but the first 4 bytes were filled automatically with NULLs followed by "56":

<?php
$str1  = 1234;
$str2  =   56;
$datei = "test.txt";

$dh = fopen($datei,"w");
fwrite($dh, $str1);
fclose($dh);

$dh = fopen ($datei,"r+");
echo "content: ".fread($dh, filesize($datei))."<br>";
echo "pointer after fread at: ".ftell($dh)."<br>";
ftruncate($dh, 0);
echo "pointer after truncate at: ".ftell($dh)."<br>";
fwrite($dh, $str2);
echo "pointer after fwrite at: ".ftell($dh)."<br>";
rewind($dh);
echo "pointer after rewind at: ".ftell($dh)."<br>";
$str = fread($dh, 6);
echo "content: $str<br>in ASCII: ";
for($i = 0; $i < 6; $i++)
 echo ord($str{$i})."-";
fclose($dh);

/*
   OUTPUT:
   content: 1234
   pointer after fread at: 4
   pointer after truncate at: 4
   pointer after fwrite at: 6
   pointer after rewind at: 0
   content: 56
   in ASCII: 0-0-0-0-53-54
*/
?>

So not only ftruncate is filling an empty file up with NULLs as in the note before. Fread is filling leading space with NULLs too.

The problem that rc at opelgt dot org mentioned seems completely logical.

When pointer is at offset 4 and you truncate file, the pointer is still at offset 4.

So when you write(), the first 4 bytes are filled with null byte by Operating System - There is nothing wrong by PHP. And it's filled with null byte, because there is data on disk and that needs to be cleared with zero bits.

Even though this is a Operating System's gotcha, to avoid data corruption, PHP Docs should mention it clearly. Also it would be nice if PHP automatically sets the pointer's offset to SEEK_END  after truncating to an smaller size to fool-proof it.

If you want to ftruncate but keep the end:
<?php
    function ftruncatestart($filename,$maxfilesize){
        $size=filesize($filename);
        if ($size<$maxfilesize*1.0) return;
        $maxfilesize=$maxfilesize*0.5; //we don't want to do it too often...
        $fh=fopen($filename,"r+");
        $start=ftell($fh);
        fseek($fh,-$maxfilesize,SEEK_END);
        $drop=fgets($fh);
        $offset=ftell($fh);
        for ($x=0;$x<$maxfilesize;$x++){
            fseek($fh,$x+$offset);
            $c=fgetc($fh);
            fseek($fh,$x);
            fwrite($fh,$c);
        }
        ftruncate($fh,$maxfilesize-strlen($drop));
        fclose($fh);
    }
?>
It will not just cut it but search for a newline so you avoid corrupting your csv or logfiles. But I don't know if you will stress the reading head of your drive. ;)