convert caps to sentese case php

PHP
//string to all uppercase
$string = "String with Mixed use of Uppercase and Lowercase";
//php string to uppercase
$string = strtoupper($string);
// = "STRING WITH MIXED USE OF UPPERCASE AND LOWERCASE"$lowercase = "this is lower case";
$uppercase = strtoupper($lowercase);

echo $uppercase;
// THIS IS LOWER CASE
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