Código PHP:
<?php
//configuro los datos de Acceso al Servidor MySql
$Host="localhost";
$User="root"; //"scjuanta_";
$Pasw="";
$Datab="caja"; //"scjuanta_innova";
$Dlink=mysql_connect($Host, $User, $Pasw);
mysql_select_db ($Datab);
?>
<html>
<head>
<title>Documento sin título</title>
<meta http-equiv="" content="text/html; charset=iso-8859-1">
</head>
<body>
<form action="<?php $_SERVER['PHP_SELF']; ?>" method="post">
<table width="250" border="0" align="center" cellpadding="0" cellspacing="1">
<tr>
<td height="26" align="right">Caja Id:</td>
<td align="left">
<?php
$consu="Select * from cajas order by id_caja";
$rst=mysql_query($consu, $Dlink);
//if ($fila=mysql_fetch_array($rst)){ $_SERVER['PHP_SELF'];
echo "<select name='mnuid' onChange="this.form.submit();">";
while ($fila=mysql_fetch_array($rst)){
if(isset($_POST['mnuid'])){
if($fila['id_caja']==$_POST['mnuid']){
echo "<option value='".$fila['id_caja']."' selected>".$fila['id_caja']."</option>";
}else{
echo "<option value='".$fila['id_caja']."'>".$fila['id_caja']."</option>";
}
}else{
echo "<option value='".$fila['id_caja']."'>".$fila['id_caja']."</option>";
}
}
echo"</select>";
mysql_free_result($rst);
if(isset($_POST['envia'])){
$consu="Select * from cajas where id_caja='".$_POST['mnuid']."'";
$rst=mysql_query($consu, $Dlink);
if ($fila=mysql_fetch_array($rst)){
$desc=$fila['descrip'];
$sig=$fila['sign'];
$sald=$fila['saldo'];
}else{
$desc="";
$sig="";
$sald="";
}
}else{
$desc="";
$sig="";
$sald="";
}
?>
</tr>
<tr>
<td align="right">Descrip:</td>
<td align="left">
<input name="Tdesc" type="text" id="Tdesc" value="<?php echo $desc; ?>" size="20" maxlength="20"></td>
</tr>
<tr>
<td align="right">Signo:</td>
<td align="left"><input name="Tsig" type="text" id="Tsig" value="<?php echo $sig; ?>" size="2" maxlength="1"></td>
</tr>
<tr>
<td align="right">Saldo:</td>
<td align="left"><input name="Tsal" type="text" id="Tsal" value="<?php echo $sald; ?>" size="15" maxlength="15"></td>
</tr>
<tr align="center">
<td colspan="2"><input name="envia" type="submit" id="envia" value="Enviar">
<input type="reset" name="Submit" value="Cancelar"></td>
</tr>
</table>
</form>
</body>
</html>
Gracias de Antemano.