/**
* version utilisant une requête xmlhttp
*/
<script type="text/javascript">
if (window.XMLHttpRequest) {
   oXmlhttp = new XMLHttpRequest();
  } else if (window.ActiveXObject) {
   oXmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
  }

function search (clef) {
 oXmlhttp.open('POST','<?php echo $_SERVER['PHP_SELF']; ?>');
 oXmlhttp.onreadystatechange=function() {
  if (oXmlhttp.readyState==4 && oXmlhttp.status == 200) {
   document.body.innerHTML = oXmlhttp.responseText;
  }
 }
 oXmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
 var data = 'liste1='+clef
 oXmlhttp.send (data);
}

</script>
<?php
$aTab = array (1 => array ('test', 'pour', 'une', 'blonde'),
  2 => array ('difficile', 'ca?', 'non...'),
  3 => array ('très', 'facile', 'en', 'fait!'));
?>

<form method="post">
<select name="liste1" onchange="search (this.value);">
<?php
foreach ($aTab as $clef => $dump) {
 $selected=(isset($_POST['liste1']) && $_POST['liste1'] == $clef)?'selected="selected"':'';
 echo '<option value="',$clef,'" ',$selected,'>',$clef,'</option>';
}
?>
</select>
<?php
if (isset ($_POST['liste1']) && !empty ($_POST['liste1'])) {
 echo '<select name="liste2">';
 foreach ($aTab[$_POST['liste1']] as $val) {
  echo '<option value="',$val,'">',$val,'</option>';
 }
 echo '</select>';
}
?>
</form>


/**
* version n'utilisant pas de requête xmlhttp
*/
<?php
$aTab = array (1 => array ('test', 'pour', 'une', 'blonde'),
  2 => array ('difficile', 'ca?', 'non...'),
  3 => array ('très', 'facile', 'en', 'fait!'));
?>

<form method="post" id="mainForm">
<select name="liste1" onchange="document.getElementById('mainForm').submit();">
<?php
foreach ($aTab as $clef => $dump) {
 $selected=(isset($_POST['liste1']) && $_POST['liste1'] == $clef)?'selected="selected"':'';
 echo '<option value="',$clef,'" ',$selected,'>',$clef,'</option>';
}
?>
</select>
<?php
if (isset ($_POST['liste1']) && !empty ($_POST['liste1'])) {
 echo '<select name="liste2">';
 foreach ($aTab[$_POST['liste1']] as $val) {
  echo '<option value="',$val,'">',$val,'</option>';
 }
 echo '</select>';
}
?>
</form>