PHP ftruncate() Function: Usage and Examples

Use ftruncate() when you need to changes the length of a file represented by an open writable stream in a PHP application. The examples below show the actual operation and its limitations.

What Is ftruncate() in PHP?

Changes the length of a file represented by an open writable stream. The exact signature supported by your installation depends on its PHP version and enabled extensions.

Its syntax is:

ftruncate($stream, int $size): bool

Parameters and return value: The parameter types, optional arguments, references, and return type are shown in the signature above. Optional parameters are marked with an ellipsis in this reference signature.

Example 1: Basic Usage

<?php

$stream = tmpfile(); fwrite($stream,"ABCDEFG"); ftruncate($stream,3); rewind($stream); echo stream_get_contents($stream); fclose($stream);

Output:

ABC

Example 2: Different Input or Behavior

<?php

$stream = tmpfile(); fwrite($stream,"123456"); ftruncate($stream,0); echo fstat($stream)["size"]; fclose($stream);

Output:

0

Practical Example: Applying the Function

<?php

$stream = tmpfile(); fwrite($stream,"old content"); ftruncate($stream,0); rewind($stream); fwrite($stream,"new"); rewind($stream); echo stream_get_contents($stream); fclose($stream);

Output:

new

Important Behavior and Edge Cases

Truncation discards data permanently on real files; pointer position is not automatically reset.

Comparison With Related Functions

Common Beginner Mistakes

Key Takeaways

• Changes the length of a file represented by an open writable stream.
• Truncation discards data permanently on real files; pointer position is not automatically reset.
• fseek() changes position without changing file length; ftruncate() changes file length.

Use ftruncate() for the specific operation demonstrated above. Check its prerequisites and handle failure conditions before using it with production data.