Se me olvidó algo, en el form haz esto:
Código PHP:
<form action="pagina.php" method="get">
<select name="multiple[]" multiple>
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
<option value="6">6</option>
<option value="7">7</option>
<option value="8">8</option>
<option value="9">9</option>
<option value="10">10</option>
</select><input name="enviar" type="submit" />
</form>