Linux apache nezapisuje do txt

<?php
      $f=fopen("xyz.txt", "a");
      $x=$_POST['x'];
      $y=$_POST['y'];
      $z=$_POST['z'];
      $a=$_POST['a'];
      $b=$_POST['b'];
      $c=$_POST['c'];
      if ($a==""||$b==""||$c==""||$x==""||$y==""||$z=="")
      {
      echo "something is wrong";
      echo "<br>"; 
      echo "<a href=add.php>Back</a>";
      }
      else
      {
      fwrite($f, $a);
      fwrite($f, "=");
      fwrite($f, $x);
      fwrite($f, ";");
      fwrite($f, $b);
      fwrite($f, "="); 
      fwrite($f, $y);
      fwrite($f, ";");
      fwrite($f, $c);
      fwrite($f, "=");
      fwrite($f, $z);      
      fwrite($f, ";");
      fwrite($f, "@");
      fclose($f);
      
      //tohle uz je jen zalohovani
      
      
      $osouboru=file_get_contents('xyz.txt');
      $promene=explode(";",$osouboru);
      $promenecount=(count($promene))-1;
      $datum=date("d_m_Y");
      if (($slovicek%6)==0)
      {
      $f = fopen("zaloha_".$datum.".txt","wb");
      fwrite($f,$osouboru);
      fclose($f);
      } 
      echo "Saved";
      echo "<br>"; 
      echo "<a href=index.php>Backn</a>";
      } 
    ?>

             $str = "$a=$x;$b=$y;$c=$z;@";
             if (!file_put_contents(__DIR__."/xyz.txt", $str, FILE_APPEND)) {
                    echo 'Data byla v pořádku, ale nezapsala se.';
             else echo 'Ok.';