Get all post

foreach( $_POST as $varname => $value ){
if( substr( $varname , 0, 3 ) == 'txt' ){
for( $i = 0 ; $i < count( $carts ); $i++ ){
$item = $carts[$i];
if( $item->id == substr( $varname , 3 ) ){
$item->quantity = $value;
$carts[$i] = $item;
}
}
}
}