Send variable php for method post - php

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']