On the second thought it doesn't seem like a feature.

"The file pointer is not changed."
(c) man ftruncate

Something like "Warning: Truncate doesn't change position of the file pointer. Adjust the pointer with fseek to write to the end of the file after truncate operation in modes other than "a" and "a+"." ?

Examples:
<?php

$fh = fopen("file.txt","w+");
fwrite($fh, "newfile");  // file contains string 'newfile'
ftruncate($fh, 3);
fseek($fh, 0, SEEK_END);
fwrite($fh, "contents"); // file contains 'newcontents'
fclose($fh);            

?>

<?php

$fh = fopen("file.txt","w+");
fwrite($fh, "newfile");  // file contains string 'newfile'
ftruncate($fh, 3);

fwrite($fh, "contents"); // file contains 'new    contents'
fclose($fh);            

?>

This bug has been fixed in the documentation's XML sources. Since the
online and downloadable versions of the documentation need some time
to get updated, we would like to ask you to be a bit patient.

Thank you for the report, and for helping us make our documentation better.

"The file pointer is changed only in append mode. In write mode, additional fseek() call is needed."

The file pointer is not changed at all by PHP when you truncate.
This is consistent with ANSI ftruncate (man ftruncate).

You are responsible for seeking explicitly if you require it.

Tnx. Now everything is clear.

The previous fix was wrong.
The docs have been updated to "The file pointer is <emphasis>not</emphasis> changed."