isset() is faster than using strlen() because calling a function is more expensive than using a language construct.

Test to show the speed http://codepad.org/ztYF0bE3

strlen() over 1000000 iterations 7.5193998813629
isset() over 1000000 iterations 0.29940009117126

Refs:

• https://www.phpreferencebook.com/tips/use-isset-instead-of-strlen/