我无法从javascript设置php变量

Question

我有这个javascript函数和html代码，我想在每种情况下设置$ _SESSION [＆＃39; gen＆＃39;]不同。但是，点击它并不重要，它总是返回最后一个，摇滚。但是，警报顺利，并返回正确的ID。

window.onload = function() {
$("a.a").click(function() {
 // alert(this.id);
  //return false;
    switch(this.id)
    {
            case 1:         
            {<?php $_SESSION['gen']='Alternative'; ?>}
            break;
            case 2:         
            {<?php $_SESSION['gen']='Blues'; ?>}
            break;
            case 3:         
            { <?php $_SESSION['gen']='Classical'; ?>}
            break;
            case 4:         
            {<?php $_SESSION['gen']='Reggae'; ?>}
            break;
            case 5:         
            {<?php $_SESSION['gen']='Electronic'; ?>}
            break;
            case 6:         
            {<?php $_SESSION['gen']='Country'; ?>}
            break;
            case 7:         
            {<?php $_SESSION['gen']='Folk'; ?>}
            break;
            case 8:         
            { <?php $_SESSION['gen']='Hip Hop'; ?>}
            break;
            case 9:         
            {<?php $_SESSION['gen']='Pop'; ?>}
            break;
            case 10:         
            {<?php $_SESSION['gen']='Rock'; ?>}
            break;
    } 
});

}

      <dl id="browse">
          <dt>Full Category Lists</dt>
          <dd class="first"><a href="genre.php" name="a" id="1" class="a">Alternative</a></dd>
          <dd><a href="genre.php" name="a" id="2" class="a">Blues</a></dd>
          <dd><a href="genre.php" name="a" id="3" class="a">Classical</a></dd>
          <dd><a href="genre.php" name="a" id="4" class="a">Reggae</a></dd>
          <dd><a href="genre.php" name="a" id="5" class="a">Electronic</a></dd>
          <dd><a href="genre.php" name="a" id="6" class="a">Country</a></dd>
          <dd><a href="genre.php" name="a" id="7" class="a">Folk</a></dd>
          <dd><a href="genre.php" name="a" id="8" class="a">Hip Hop</a></dd>
          <dd><a href="genre.php" name="a" id="9" class="a">Pop</a></dd>
          <dd class="last"><a href="genre.php" name="a" id="10" class="a">Rock</a></dd>
      </dl>