problema lettura scrittura su file

[bobiron]

L'output dello script seguente non da il risultato sperato "123456" ma invece "1234".
Non riesco a capire perchè!!

Grazie in anticipo
Roberto

<?php
  $fname = "test.txt";
  
  $fd = fopen( $fname, "w");
  fwrite( $fd, "1234");
  fclose( $fd);
  
  $fd = fopen( $fname, "r");
  $txt = fread( $fd, filesize( $fname));
  fclose( $fd);
  
  $txt .= "56"; 

  $fd = fopen( $fname, "w");
  fwrite($fd, $txt);
  fclose( $fd);
  
  $fd = fopen( $fname, "r");
  $txt = fread( $fd, filesize( $fname));
  fclose( $fd);
    
  echo $txt;  
?>