Código PHP:
<div class="boton"><input type="submit" name="boton" value="enviar" /></div>
<br />
</form>
<?php if (isset($_POST["boton"])) {
$a = $_POST['a1'];
$b = $_POST['a2'];
$c = $_POST['a3'];
$d = $_POST['a4'];
$e = $_POST['a5'];
$f = $_POST['a6'];
$g = $_POST['a7'];
$h = $_POST['a8'];
$i = $_POST['a9'];
$j = $_POST['a10'];
$k = $_POST['a11'];
$l = $_POST['a12'];
$m = $_POST['a13'];
$n = $_POST['a14'];
$o = $_POST['a15'];
$p = $_POST['a16'];
$q = $_POST['a17'];
$r = $_POST['a18'];
$s = $_POST['a19'];
$t = $_POST['a20'];
$u = $_POST['sexo'];
$v = $_POST['campus'];
$w = $_POST['titulo'];
$y = $_POST['curso'];
$z = $_POST['edad'];
if ($w=="Seleccione" || $v=="Seleccione" || $z=="" || $u=="" || $a=="" || $b=="" || $c=="" || $d=="" || $e=="" || $f=="" || $g=="" || $h=="" || $i=="" || $j=="" || $k=="" || $l=="" || $m=="" || $n=="" || $o=="" || $p=="" || $q=="" || $r=="" || $s=="" || $t=="" ) {
echo "<script>alert('Algun campo no ha sido rellenado');</script>";
}
else {