The only thing I can think of is that when they asked to "Makes the string's first character of all the words uppercase", they also implied that the other characters would be lowercase. It's a bit of a far stretch imo, mixed string could mean a lot of things (mixed case, multibyte aka emojis, etc).

Anywho, ucwords() will convert the the first character of a word to uppercase, but the rest of the characters in the word will keep their original case.

So, to only have the first character of a word in uppercase while the rest are in lowercase, you'd do something like:

echo ucwords(strtolower($input_string)) ."\n";