To convert a string from CamelCase to snake_case in PHP, you can use the following function:

function camelCaseToSnakeCase($input) {
    return strtolower(preg_replace('/(?<=\\w)([A-Z])/', '_\\1', $input));
}

Here’s how the function works:

1. (?<=\\w) is a positive lookbehind that ensures there is a word character (letter, digit, or underscore) before the match. This is to avoid inserting underscores at the beginning of the string.
2. ([A-Z]) is a capturing group that matches any uppercase letter (A to Z).
3. '_\\1' is the replacement pattern, which inserts an underscore followed by the first capturing group (the uppercase letter) where the match occurred.

Let’s see some examples:

echo camelCaseToSnakeCase('helloWorld'); // Output: "hello_world"
echo camelCaseToSnakeCase('helloWorldExample'); // Output: "hello_world_example"
echo camelCaseToSnakeCase('anotherExample123'); // Output: "another_example123"

The function will convert the CamelCase input to snake_case as expected. Note that numbers are not affected, and only uppercase letters are replaced with underscores followed by the lowercase version of the letter.