<?php
function decamelize($string) {
	return strtolower(preg_replace(['/([a-z\d])([A-Z])/', '/([^_])([A-Z][a-z])/'], '$1_$2', $string));
}

$tests = array(
	'simpleTest'      => 'simple_test',
	'easy'            => 'easy',
	'HTML'            => 'html',
	'simpleXML'       => 'simple_xml',
	'PDFLoad'         => 'pdf_load',
	'startMIDDLELast' => 'start_middle_last',
	'AString'         => 'a_string',
	'Some4Numbers234' => 'some4_numbers234',
	'TEST123String'   => 'test123_string',
	'hello_world'     => 'hello_world',
	'hello___world'   => 'hello___world',
	'_hello_world_'   => '_hello_world_',
	'HelloWorld'      => 'hello_world',
	'helloWorldFoo'   => 'hello_world_foo',
	'hello_World'     => 'hello_world',
	'hello-world'     => 'hello-world',
	'myHTMLFiLe'      => 'my_html_fi_le',
	'aBaBaB'          => 'a_ba_ba_b',
	'BaBaBa'          => 'ba_ba_ba',
	'libC'            => 'lib_c',
);

foreach ($tests as $test => $result) {
	$output = decamelize($test);
	if ($output === $result) {
		echo "Pass: $test => $result\n";
	} else {
		echo "Fail: $test => $result [$output]\n";
	}
}

Code

Result

PHP function to convert a string from CamlCase to snake_case

This is a very simple PHP function (only one single line) to convert a string from CamlCase to snake_case.

All these use cases are handled:

simpleTest => simple_test
easy => easy
HTML => html
simpleXML => simple_xml
PDFLoad => pdf_load
startMIDDLELast => start_middle_last
AString => a_string
Some4Numbers234 => some4_numbers234
TEST123String => test123_string
hello_world => hello_world
HelloWorld => hello_world
helloWorldFoo => hello_world_foo
hello_World => hello_world
hello-world => hello-world
myHTMLFiLe => my_html_fi_le
aBaBaB => a_ba_ba_b
BaBaBa => ba_ba_ba
libC => lib_c