Gerardo haciendo lo que me dijiste obtengo esto...
Código HTML:
<br />
<b>Notice</b>: Undefined variable: id_checkbox in <b>/var/www/programacion1/TP3/ejercicio3/cargar.php</b> on line <b>3</b><br />
<br />
<b>Notice</b>: Undefined index: cant_ in <b>/var/www/programacion1/TP3/ejercicio3/cargar.php</b> on line <b>3</b><br />
Array
(
[cant_1] =>
[cant_2] =>
[cant_3] =>
[cant_4] =>
[cant_5] =>
[cant_6] =>
[seleccion] => Array
(
[0] => 7
)
[cant_7] => 4
[cant_8] =>
[cant_9] =>
[cant_10] =>
[cant_11] =>
[cant_12] =>
[cant_13] =>
[cant_14] =>
[cant_15] =>
[cant_16] =>
[cant_17] =>
[cant_18] =>
[cant_19] =>
[cant_20] =>
[cant_21] =>
[cant_22] =>
[cant_23] =>
[cant_24] =>
[cant_25] =>
[cant_26] =>
[cant_27] =>
[cant_28] =>
[cant_29] =>
[cant_30] =>
[cant_31] =>
[cant_32] =>
[cant_33] =>
[cant_34] =>
[cant_35] =>
[boton] => Cargar
)
Puse $_POST["cant_$id_checkbox"] tanto en el formulario como en la parte de levantar la variable.