When writing to a file, you should avoid using w+ because it would erase the contents of the file before locking
If you need to write the complete file again you could use the following instead:
<?php
$fp = fopen('yourfile.txt', 'a') ;
if (flock($fp, LOCK_EX)) {
 ftruncate($fp, 0) ; // <-- this will erase the contents such as 'w+'
 
 fputs($fp, 'test string') ;
 
 flock($fp, LOCK_UN) ;
}
fclose($fp) ;
?>

Best,
Fernando Gabrieli