Hack

For all practical purposes it is impossible to hack an iPhone without physical possession of the phone while it is unlocked.

Hackers CAN get into your Apple ID account if you don’t have 2 factor authentication and also have a weak password. If that is a possibility change your password immediately, add 2 factor authentication, and read this link→If you think your Apple ID has been compromised - Apple Support. And anyone with access to your Apple ID can steal your identity, and take over your phone’s account, including locking it.

Anything that is not "air-gapped" meaning it is connected to a network directly like your iPhone can be hacked. Though, The iPhone is undoubtedly more resistant than the competition when it comes to hacking.

Apple keeps tight control over both software and hardware, resulting in a secure device. However, certain things can pierce your iPhone’s security. 

I will explain for you to gain a better understanding of what can affect your iPhone’s security (and user experience):

Jailbreaking 

If you jailbreak your device or run an enterprise developer account, you have the option to install apps outside of the App Store. This may (unknowingly) result in malicious apps taking place on your iPhone. In short, never jailbreak!

Accessing unknown links

Your iPhone can be compromised if you open harmful phishing links you receive on iMessage or email from anonymous people. If you are unsure of a link to not click it. Believe nothing is that important and you did not win a gift card!

Downloading apps from unknown sources

Apple is strict about the App Store rules and does not allow any malicious apps or existing app updates that may compromise your device’s security. Additionally, you cannot download apps from anywhere else except the official App Store.

These two factors bring down the risk of being troubled by a third-party app to a minimal level. But still, in rare cases, an app may get through approval with malicious code. 

Additionally, installing random third-party keyboards and giving them full access can also affect your iPhone’s security.

EASIEST WAY FOR ME TO PENETRATE AN IPHONE IS Public charging stations/Wi-Fi

Using untrusted public charging stations might be the culprit (especially if your iPhone is unlocked). Be tech-savvy and add passcodes and keep your phone locked tight. Also, using free public Wi-Fi for bank transactions (and similar data-sensitive procedures) on your iPhone can prove disastrous.

Has your iPhone been hacked? Here’s how to check

• You may see new apps on the Home Screen that you don’t recognize.
• There may be a severe battery drain.
• The iPhone may run hot and open apps randomly.
• Your iPhone is exchanging an abnormal amount of data.
• The keyboard may misbehave and type random words.
• Call, message, and email history may have unknown entries that you didn’t make.
• You may see random popups on your iPhone.

If you notice one or more of these points, there’s a good chance some malicious code landed itself on your iPhone. (Your phone may run hot when under stress, this is normal. If your iPhone is hot when you don’t have CPU-intensive apps open, its cause for concern.)

4 Ways to fix a hacked iPhone

The combination of iOS and Apple’s secure hardware offers a safe user experience. But in rare times, you may still fall prey to an attack.

Here’s what to do to safeguard yourself and your device.

Turn off the internet

Many hacks are controlled via the web. If you have reason to believe that your iPhone has been compromised, the first thing to do is turn off Wi-Fi and cellular data. Try putting the phone in AirPlane mode, and eject the SIM card.

1. Open Settings on your iPhone.
2. Tap Wi-Fi.
3. From the next screen, turn it off.
4. Go back, and tap Cellular. Turn it off as well.

Delete any unrecognized apps

If you see a new app you don’t remember installing, long-press it and delete it.

Has your iPhone been hacked? Here’s how to check

• You may see new apps on the Home Screen that you don’t recognize.
• There may be a severe battery drain.
• The iPhone may run hot and open apps randomly.
• Your iPhone is exchanging an abnormal amount of data.
• The keyboard may misbehave and type random words.
• Call, message, and email history may have unknown entries that you didn’t make.
• You may see random popups on your iPhone.

trxjcb wrote:

Are iphones easily hacked by hackers?

No. Not if you don't jailbreak the phone.

Can hackers lock your iphone so you will not be able to open it?

Not if you don't let them have your Apple ID credentials.

RevoltAnon wrote:

Accessing unknown links

Your iPhone can be compromised if you open harmful phishing links you receive on iMessage or email from anonymous people. If you are unsure of a link to not click it. Believe nothing is that important and you did not win a gift card!

Overall a good analysis, except for the above. An iPhone that is fully updated cannot be compromised by opening a phishing link unless you provide information after opening the link.

Thanks for this comprehensive answer.