$fh = fopen('myfile.txt', 'w+');
$content = fread( $h, filesize('myfile.txt'));
echo $content;
fseek( $fh, 0 );
fwrite( $fh, $newcontent );
ftruncate( $fh, strlen( $newcontent ) );

No matters de contents of myfile.txt, no output is generated. 
If I replace the first line to 
$fh = fopen('myfile.txt', 'r+');
everythings works fine if the file exists.

Manu.