Update: Whatsapp Web is officially available for iPhone. This workaround to enable WhatsApp Web for iPhone was more appropriate in the past when WhatsApp was not available on iPhone.
Original Article continues Below:
As of now, there is an official way to access WhatsApp on an iPhone. However, there is an option to allow this feature on the WhatsApp Web directly from your iPhone by using the WhatsApp Web Enabler that works on jailbroken iPhone.
WhatsApp Web Enabler
WhatsApp Web Enabler was developed to work around this issue by developers IMokhles to allow the WhatsApp Web application for the iPhone. There is no official method to access WhatsApp Web via an iPhone and it is some way to go until WhatsApp Web becomes available for iPhone.
WhatsApp Web Enabler can be downloaded for no cost from the BigBoss repository on Cydia. The developer has provided the option to disable or activate the tweak in the Settings app on your iPhone.
How do I use WhatsApp Web using iPhone
1. Download the most recent version of WhatsApp for iOS and install it on your iPhone.
2. You can download Chrome, Firefox or Opera browsers on your personal computer. At present, WhatsApp Web only works with these browsers, and is available on Safari (MacOS 10.8plus).
3. Installation of WhatsApp Web Enabler for your jailbroken iPhone You’ll need to start Cydia and search at the WhatsApp Web Enabler mod. You will then apply it onto your Phone.
4. In the Settings application, you’ll see an option for ‘WhatsApp Web’ that was added to the app.
5. Look up the preferences of WhatsApp Web Enabler, and turn on the modification.
6. Force-close or close or quit WhatsApp and then reopen it.
7. Select the Settings tab, choose WhatsApp Web before tapping “OK. I got it.”
8. From you desktop computer (not the iPhone) visit web.whatsapp.com in your web browser. Like I said, the browser must have Chrome, Firefox or Opera.
9. When you arrive at web.whatsapp.com There will be an QR code. (see below)
10. You can scan your QR Code by the iPhone towards the computer screen.
11. After your phone has scanned the code, you’ll discover that you are automatically signed into WhatsApp. WhatsApp Web.
12. Text message messages from your computer to confirm you are sure that WhatsApp web is running on your system.