(PHP 4, PHP 5, PHP 7)
ftruncate — Truncates a file to a given length
ftruncate ( resource $handle , int $size ) : bool
 Takes the filepointer, handle, and truncates the file to length, size. 
handle
The file pointer.
Note:
The
handlemust be open for writing.
size
The size to truncate to.
Note:
If
sizeis larger than the file then the file is extended with null bytes.If
sizeis smaller than the file then the file is truncated to that size.
 Returns true on success or false on failure. 
Example #1 File truncation example
<?php $filename = 'lorem_ipsum.txt'; $handle = fopen($filename, 'r+'); ftruncate($handle, rand(1, filesize($filename))); rewind($handle); echo fread($handle, filesize($filename)); fclose($handle); ?>
Note:
The file pointer is not changed.
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    https://www.php.net/manual/en/function.ftruncate.php