PHP - add underscores before capital letters

Question

How can I replace a set of words that look like:

SomeText

to

Some_Text

?

Answer 1

$result = strtolower(preg_replace('/(.)([A-Z])/', '$1_$2', $subject));

Converts:

HelloKittyOlolo
Declaration
CrabCoreForefer
TestTest
testTest

To:

hello_kitty_ololo
declaration
crab_core_forefer
test_test
test_test