Unfortunately iOS limitations creates obstacles in publishing a WhatsApp version for iPhone users. In future there would be a version for iOS, but for now if your phone happens to be jailbroken then we can get around this.
How to Enable WhatsApp Web for iPhone
Step 1: Jailbreak your device. (Look for the iOS version you have in the search results)
Step 2: Head over to Cydia and search for: WhatsApp Web Enabler. Install it.
Step 3: Now, go to Settings, scroll down and tap on WhatsApp Web Enabler, find the Enable/Disable switch and make sure its Enabled.
Step 4: Launch WhatsApp, tap on Settings from the bottom right corner, tap on WhatsApp Web
Step 5: On your computer open web.whatsapp.com site and using your iPhone scan the QR code, when it completes scanning you’ll be able to use WhatsApp Web like Android, Windows and Blackberry users.