how to take a screenshot of the iPhone programmatically?

In modern way:

Obj-C

@interface UIView (Snapshot)
- (UIImage * _Nullable)snapshot;
@end

@implementation UIView (Snapshot)

- (UIImage * _Nullable)snapshot {
    UIGraphicsBeginImageContextWithOptions(self.bounds.size, NO, UIScreen.mainScreen.scale);
    [self drawViewHierarchyInRect:self.bounds afterScreenUpdates:YES];
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return image;
}

@end

Swift

extension UIView {
    func snapshot() -> UIImage? {
        UIGraphicsBeginImageContextWithOptions(bounds.size, false, UIScreen.main.scale)
        drawHierarchy(in: bounds, afterScreenUpdates: true)
        let image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return image
   }
}