A pretty simple, light, and though efficient solution I frequently use in such cases is to merely:

• Return the appropriate explainatory message when there is an error.
• Do nothing otherwise.

First it makes your function as simple as possible (when successfull you don't even need to return anything).

Then from the app which invokes the function it's easy to use while being readable, assumed you change the function name for something like invalidUser(). So the app can work like this:

if ($errorMessage = invalidUser(...)) {
  // take any appropriate action, using $errorMessage
}
// otherwise merely continue

Note: the suggestion below is founded on a certain vision about readability, that may not be shared by everybody, as illustrated by the totally opposite approach proposed by @Thijs Riezebeek.

A pretty simple, light, and though efficient solution I frequently use in such cases is to merely:

• Return the appropriate explainatory message when there is an error.
• Do nothing otherwise.

First it makes your function as simple as possible (when successfull you don't even need to return anything).

Then from the app which invokes the function it's easy to use while being readable, assumed you change the function name for something like invalidUser(). So the app can work like this:

if ($errorMessage = invalidUser(...)) {
  // take any appropriate action, using $errorMessage
}
// otherwise merely continue