1. Ensure both Macbook Pro and iPhone are on the same wireless network.
2. Verify Macbook Pro IP address, System Preferences > Network > Wi-Fi > Status (ie. 10.61.0.125).
3. Open apache httpd.conf file, Applications > MAMP > conf > apache.
4. On httpd.conf file, Command+F Listen to verify the port assigned (ie. 8888).
5. Open iPhone browser (safari/chrome, etc.).
6. Type the IP address and port assignment, 10.61.0.125:8888.

Thanks to Stackoverflow

PS: This also holds true for ANDROID phones with their respective browsers. As long as you are in the same network.