Código HTML:
<form id="form1" name="form1" method="post" action="BDD contratacion.php">
<label for="select3">Ultimas Contrataciones</label>
<select name="select" id="select" onchange = "this.form.submit()" >
<option value="Dia">Dia</option>
<option value="Semana">Semana</option>
<option value="Quincena">Quincena</option>
<option value="Mes">Mes</option>
</select>
</form> Código PHP:
$result = false;
if( isset($_POST['select']) && !empty($_POST['select']) ){
$result = mysql_query("SELECT Email, FROM contratacion where Tiempo_Contratado='".$_POST['select']."'");
}