How to convert CamelCase to snake_case in PHP (in a simpler way)?

Ready with a line has this one from the SOzão

$saida = ltrim(strtolower(preg_replace('/[A-Z]/', '_$0', $entrada )), '_');

The question is: What happens if you have ___ at the beginning?

It may be interesting to change the logic if it is always to start with a letter:

$saida = substr(strtolower(preg_replace('/[A-Z]/', '_$0', $entrada )), 1);

This here already guarantees the first character does not have _ using lcfirst :

$saida = strtolower( preg_replace(
    ["/([A-Z]+)/", "/_([A-Z]+)([A-Z][a-z])/"], ["_$1", "_$1_$2"], lcfirst($entrada) ) );

Remarks:

• See these situations:

  LetraEMaiuscula 
  CodeIsHTML
  

  In the first case, using [a-z][A-Z] gives problem. In the second case, [a-z][A-Z] is better than just [A-Z] . If you need the second case you have to use an exchange in the format ([A-Z])([A-Z][a-z]) by $1_$2

• What if I have spaces? I believe that if it is already well-formed CamelCase, it does not make sense to have spaces in the string, but if you need to change by _ , it is the case to add a str_replace(' ', '_', $valor ) as already noted Miguel in