My preferred is debug from https://github.com/hazardland/debug.php which is library with containing only single function named as debug (You can just copy that function in your project or in your library). Typical debug() html output looks like this:
But you can output data as a plain text with same function also (with 4 space indented tabs) like this (and even log it in file if needed):
string : "Test string"
boolean : true
integer : 17
float : 9.99
array (array)
bob : "alice"
1 : 5
2 : 1.4
object (test2)
another (test3)
string1 : "3d level"
string2 : "123"
complicated (test4)
enough : "Level 4"
