// JavaScript Document
var d   = document;
var req = Create();

function orarios() 
{
  var a = d.getElementById("cursos").value;
  query = "cursos="+a;
  Request(query);
}

function Create()
{
  if ( navigator.appName == "Microsoft Internet Explorer" )
    req = new ActiveXObject( "Microsoft.XMLHTTP" );
  else
    req = new XMLHttpRequest();
  return req;
}

function Request( query ) 
{
  req.open("post", "function.php", true );
  req.onreadystatechange = Refresh;
  req.setRequestHeader("Content-Type","application/x-www-form-urlencoded; charset=utf-8");
  req.send(query);
}


function Refresh() 
{
  if( req.readyState == 4 )
  d.getElementById("orario").innerHTML = req.responseText;
}

<p class="formulario2"><label><span class="input"> Cursos que Interesan </span>
         <select name="cursos" id="cursos" onchange="orarios();" style="width: 253px;">
           <option value="0">---Seleccione un curso---</option>
           <option value="1">Office nivel b&aacute;sico + S.O.</option>
           <option value="2">Office nivel medio/avanzado</option>
           <option value="3">Internet y correo electr&oacute;nico</option>
           <option value="4">Idiomas ( ingl&eacute;s, franc&eacute;s )</option>
           <option value="5">Cursos a medida bajo demanda</option>
           <!--<option value="5">Otros Cursos</option>-->
         </select>
         </label>
      </p>
      <p id="orario" class="formulario2"><label>
     	<?php include('function.php'); ?>
      </label></p>

<?php
function orarios() {

if ( $_POST['cursos'] < 4 ) {

	$array = array( 
								'1' => array( 
														'bla bla bla bla',
														'11:00 - 12:30 (L y X)', 
														'11:00 - 12:30 (M y J)', 
														'17:30 - 19:00 (L y X)', 
														'17:30 - 19:00 (M y J)'
								),
								'2' => array(
														'bla bla bla bla',
														'19:00 - 20:30 (L a J)'
								),
								'3' => array(
														'bla bla bla bla',
														'12:30 - 14:00 (L y X)',
														'12:30 - 14:00 (M y J)',
														'16:00 - 17:30 (L y X)',
														'16:00 - 17:30 (M y J)'
								),
	);

	$arr =  $array[ $_POST['cursos'] ];
	$cnt = count( $arr );
	
	$select = '<span class="input"> Cursos que Interesan </span>';
	$select .= '<select name="orarios">';
	
	foreach ( $arr as $key => $val ) {
	
	  $select .= '<option value ="'.$key.'">'.$val.'</option>';
	
	}
	
	$select .= '</select>';     
	return $select;
	
} else {
	
	return false;
	
}

}
if ( isset( $_POST['cursos'] ) )
echo orarios();
?>