Today I stumbled on a bug report for the Mail_Mime package in PEAR: Bug #20222: 1.8.8 not compatible with PHP4 .
So in March 2014, someone noticed that a minor version upgrade of a package broke it on PHP4. Whoa.
But PEAR takes backwards compatibility very serious , so this bug was fixed.
The PEAR version naming standard looks similar to semantic versioning. Just remember that PEAR already decided on 2004-11-21 to follow this rules.