Si eso se hace con javascript te dejo este ejemplo
Código PHP:
<html>
<head>
<!-- Copiar dentro del tag HEAD -->
<script LANGUAGE="JavaScript">
function Validar(form)
{
if (form.Nombre.value == "")
{ alert("Por favor ingrese su nombre"); form.Nombre.focus(); return; }
if (form.Email.value == "")
{ alert("Por favor ingrese su dirección de e-mail"); form.Email.focus(); return; }
if (form.Domicilio.value == "")
{ alert("Por favor ingrese su domicilio"); form.Domicilio.focus(); return; }
if (form.Telefono.value == "")
{ alert("Por favor ingrese su número de teléfono"); form.Telefono.focus(); return; }
if (form.Empresa.value == "")
{ alert("Por favor ingrese el nombre de su empresa"); form.Empresa.focus(); return; }
if (form.NumeroTarjeta.value == "")
{ alert("Por favor ingrese los números de su tarjeta de crédito"); form.NumeroTarjeta.focus(); return; }
if (form.Codigo.value == "")
{ alert("Por favor ingrese el código de su tarjeta de crédito"); form.Codigo.focus(); return; }
if (form.NombreTitular.value == "")
{ alert("Por favor indique el nombre del titular de la tarjeta de crédito"); form.NombreTitular.focus(); return; }
if (form.Email.value.indexOf('@', 0) == -1 ||
form.Email.value.indexOf('.', 0) == -1)
{ alert("Dirección de e-mail inválida"); form.Email.focus(); return; }
form.submit();
}
</script>
</head>
<body>
<!-- Copiar dentro del tag BODY -->
<form METHOD="post" ACTION="../cgi-bin/formulario.cgi">
<div align="center"><center><table border="0" width="100%">
<tr>
<td width="20%" align="right"><font face="Arial" color="#000000"><small>Apellido</small></font></td>
<td width="30%"><font color="#FFFFFF" face="Arial"><input type="text" name="Apellido"
size="20"></font></td>
<td width="20%" align="right"><div align="right"><p><font face="Arial" color="#000000"><small>Nombre</small></font></td>
<td width="30%"><font color="#FFFFFF" face="Arial"><input type="text" name="Nombre"
size="20"></font></td>
</tr>
<tr>
<td width="20%" align="right"><font face="Arial" color="#000000"><small>Domicilio</small></font></td>
<td width="30%"><font color="#FFFFFF" face="Arial"><input type="text" name="Domicilio"
size="20"></font></td>
<td width="20%" align="right"><font face="Arial" color="#000000"><small>(*) Ciudad</small></font></td>
<td width="30%"><font color="#FFFFFF" face="Arial"><input type="text" name="Ciudad"
size="20"></font></td>
</tr>
<tr>
<td width="20%" align="right"><font face="Arial" color="#000000"><small>(*) Provincia</small></font></td>
<td width="30%"><font color="#FFFFFF" face="Arial"><input type="text" name="Provincia"
size="20"></font></td>
<td width="20%" align="right"><font face="Arial" color="#000000"><small>(*) Codigo Postal</small></font></td>
<td width="30%"><font color="#FFFFFF" face="Arial"><input type="text" name="CodPost"
size="20"></font></td>
</tr>
<tr>
<td width="20%" align="right"><font face="Arial" color="#000000"><small>(*) Pais</small></font></td>
<td width="30%"><font color="#FFFFFF" face="Arial"><input type="text" name="Pais"
size="20"></font></td>
<td width="20%" align="right"><font face="Arial" color="#000000"><small>E-mail</small></font></td>
<td width="30%"><font color="#FFFFFF" face="Arial"><input type="text" name="Email"
size="20"></font></td>
</tr>
<tr>
<td width="20%" align="right"><font face="Arial" color="#000000"><small>Teléfono</small></font></td>
<td width="30%"><font color="#FFFFFF" face="Arial"><input type="text" name="Telefono"
size="20"></font></td>
<td width="20%" align="right"><font face="Arial" color="#000000"><small>(*) Fax</small></font></td>
<td width="30%"><font color="#FFFFFF" face="Arial"><input type="text" name="Fax" size="20"></font></td>
</tr>
<tr>
<td width="20%" align="right"></td>
<td width="30%"></td>
<td width="20%" align="right"><font face="Arial" color="#000000"> </font></td>
<td width="30%"></td>
</tr>
<tr>
<td width="20%" align="right"><font face="Arial" color="#000000"><small>Empresa</small></font></td>
<td width="30%"><font color="#FFFFFF" face="Arial"><input type="text" name="Empresa"
size="20"></font></td>
<td width="20%" align="right"><font face="Arial" color="#000000"><small>(*) Dirección</small></font></td>
<td width="30%"><font color="#FFFFFF" face="Arial"><input type="text"
name="DireccionEmpresa" size="20"></font></td>
</tr>
<tr>
<td width="20%" align="right"><font face="Arial" color="#000000"><small>(*) Teléfono</small></font></td>
<td width="30%"><font color="#FFFFFF" face="Arial"><input type="text"
name="telefonoEmpresa" size="20"></font></td>
<td width="20%" align="right"><font face="Arial" color="#000000"><small>(*) Fax</small></font></td>
<td width="30%"><font color="#FFFFFF" face="Arial"><input type="text" name="FaxEmpresa"
size="20"></font></td>
</tr>
<tr>
<td width="20%" align="right"><font face="Arial" color="#000000"><small>(*) E-mail</small></font></td>
<td width="30%"><font color="#FFFFFF" face="Arial"><input type="text" name="EmailEmpresa"
size="20"></font></td>
<td width="20%" align="right"></td>
<td width="30%"></td>
</tr>
<tr>
<td width="20%" align="right"></td>
<td width="30%"></td>
<td width="20%" align="right"></td>
<td width="30%"></td>
</tr>
<tr>
<td width="20%" align="right"><font face="Arial" color="#000000"><small>Tarjeta de Credito</small></font></td>
<td width="30%"><font color="#FFFFFF" face="Arial"><select name="Tarjeta" size="1">
<option value="Amex">Amex</option>
<option value="Visa">Visa</option>
<option value="Mastercard">Mastercard</option>
<option value="Diners">Diners</option>
</select></font></td>
<td width="20%" align="right"><font face="Arial" color="#000000"><small>Nombre del Titular</small></font></td>
<td width="30%"><font color="#FFFFFF" face="Arial"><input type="text" name="NombreTitular"
size="20"></font></td>
</tr>
<tr>
<td width="20%" align="right"><font face="Arial" color="#000000"><small>Numero</small></font></td>
<td width="30%"><font color="#FFFFFF" face="Arial"><input type="text" name="NumeroTarjeta"
size="20"></font></td>
<td width="20%" align="right"><font face="Arial" color="#000000"><small>Código de
seguridad</small></font></td>
<td width="30%"><font color="#FFFFFF" face="Arial"><input type="text" name="Codigo"
size="5"></font></td>
</tr>
<tr>
<td width="20%" align="right"><font face="Arial" color="#000000"><small>(*) Fecha de
vencimiento</small></font></td>
<td width="30%"><font color="#FFFFFF" face="Arial"><input type="text"
name="VencimientoMes1" size="2"> / <input type="text" name="Vencimientoano1" size="2"></font></td>
<td width="20%" align="right"></td>
<td width="30%"></td>
</tr>
<tr>
<td width="20%" align="right"></td>
<td width="30%"></td>
<td width="20%" align="right"></td>
<td width="30%"></td>
</tr>
<tr>
<td width="100%" align="center" colspan="4"><div align="center"><center><table border="0"
width="100%" cellspacing="15" cellpadding="0">
<tr>
<td width="100%"><div align="center"><center><p><font face="Arial" color="#000000"><small><input
TYPE="button" VALUE="Enviar" onClick="Validar(this.form)"><input TYPE="reset"></small></font></td>
</tr>
</table>
</center></div></td>
</tr>
</table>
</center></div>
</form>
</body>
</html>