Send variable php for method post

Question

I get a var called $cobro in this module php, and i need this variable for calculate a subtraction of a number insert in a input called $vuelto. When i send the form the var $cobro is eliminated and i cant execute the subtraction.

Help, and thx !

<?php
// Disponible desde PHP 4.1.0
date_default_timezone_set("America/Santiago");

if(isset($_POST['fecha'])){
   $fecha = date("Y-m-d G:i:s",strtotime($_POST['fecha']));
}

$date1 = strtotime($fecha);
$date2 = time();

$subTime = $date2-$date1;
$y = ($subTime/(60*60*24*365));
$d = ($subTime/(60*60*24))%365;
$h = ($subTime/(60*60))%24;
$m = ($subTime/60)%60;

echo "Diferencia entre ".$fecha." y ".date('Y-m-d H:i:s',$date2)." es:<br/>";
//echo $y." annos<br/>";
echo $d." dias<br/>";
echo $h." horas<br/>";
echo $m." minutos<br/>";

$cobro =0;

$fecha_em= date('Y-m-d H:i:s',$date2);

$h = $h + $m/60 +$d*24;
$cobro = $h*600;

if($cobro<0)
    $cobro = $cobro*-1;

if($h < 1)
   echo "Debe pagar el minimo: $600";
else
   echo "Debe pagar: $".$cobro;

$conexion = mysql_connect("localhost","grupo2","face2014");

mysql_select_db("sisace",$conexion); 

/*if ($conexion==0)
echo "Lo sentimos, no se ha podido conectar con la MySQL";
else {
echo "Se logró conectar con MySQL";
echo "<br>";}
*/

$sql="INSERT INTO boleta(fecha_emision,fecha_ingreso,num_boleta,valor_total) ".
     "VALUES('$fecha_em','$fecha','1','$cobro')";

mysql_query($sql);

if(isset($_POST['submit'])) {
   $vuelto = $_POST['vuelto'];
   $vuelto = $vuelto -$cobro;
   echo "Su vuelto es el siguiente:";
   echo $vuelto;
}
?>

<form method="post" action="<?php echo $_SERVER['PHP_SELF']; ?>">
  <input type="number" name="vuelto"><br>
  <input type="submit" name="submit" value="Submit Form"><br>
</form>

Answer 1

You didn't put it in Form use input hidden to post it

<form method="post" action="<?php echo $_SERVER['PHP_SELF']; ?>">
   <input type="number" name="vuelto"><br>
   <input type="hidden" name="cobro" value="<?php echo $cobro; ?>">
   <input type="submit" name="submit" value="Submit Form"><br>
</form>

That's how you can pass cobro variable value which will be available in $_POST['cobro']