The best replacement for user agent is feature detection.

The best alternatives to user agent are rated as following:
★★★★★  feature detection
★★  x mailer

Your alternative for user agent is not listed? Suggest!
Speaking about user agent, one probably shouldn't forget about curl.

Other search terms related to user agent and their ratings are:
★★★★★ curl
★★ curl in php
★ wget
★ file_get_contents
★ mobile browser
You have in mind a companion for user agent and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of user agent in all years. Click on graph for more.
user agent
feature detection
x mailer