Código PHP:
<html>
<head>
<script language="JavaScript">
function noVacio() {
var i;
var n = parseInt(document.frm.cantidad.value);
var bError = false;
for (i = 0; i < n; i++){
bError = bError || (eval("document.frm.elemento" + i + ".value == ''"));
if (bError){
alert("Campo inválido");
eval("document.frm.elemento" + i + ".focus()");
break;
}
}
if (!bError) alert("Ok");
}
</script>
</head>
<body>
<form name="frm">
<table width="75%" border="0">
</table>
<table width="25%" border="0">
<tr>
<td>Talla</td>
<td><select name="select">
<option>Grande</option>
<option>Mediana</option>
<option>Chica</option>
</select></td>
</tr>
<tr>
<td>Color</td>
<td><select name="select2">
<option>Negro</option>
<option>Blanco</option>
<option>Rojo</option>
<option>Azul</option>
<option>Amarillo</option>
</select></td>
</tr>
<tr>
<td height="24">Estilo </td>
<td><select name="select3">
<option>Polo</option>
<option>T-shirt</option>
<option>Cuello V</option>
<option>Manga Larga</option>
<option>Sudadera</option>
</select></td>
</tr>
<tr>
<td height="24">Cantidad</td>
<td><select name="select3">
<option>1</option>
<option>2</option>
<option>3</option>
<option>4</option>
<option>5</option>
<option>6</option>
<option>7</option>
<option>8</option>
<option>9</option>
<option>10</option>
</select></td>
</tr>
<tr>
<td height="24">Factura</td>
<td><select name="select4">
<option>Si</option>
<option>No</option>
</select></td>
</tr>
<tr>
<td height="24">RFC</td>
<td><input name="textfield" type="text" maxlength="13"></td>
</tr>
</table>
<div align="left"><br>
<input type="hidden" name="cantidad" value="3">
<input type="button" name="enviar" value="enviar" onclick="noVacio()">
</div>
</form>
</body>
</html>
