How to capitalize first letter of first word in a sentence?

Question

I am trying to write a function to clean up user input.

I am not trying to make it perfect. I would rather have a few names and acronyms in lowercase than a full par

Answer 1

$output = preg_replace('/([.!?])\s*(\w)/e', "strtoupper('\\1 \\2')", ucfirst(strtolower($input)));

Since the modifier e is deprecated in PHP 5.5.0:

$output = preg_replace_callback('/([.!?])\s*(\w)/', function ($matches) {
    return strtoupper($matches[1] . ' ' . $matches[2]);
}, ucfirst(strtolower($input)));