"or does anyone have a solution?"

^ By that, I take it you might be looking for an alternate solution.

In order to keep the original value, you can assign it to a variable, and use strtolower() to make it lowercase, then strtoupper() to make it uppercase.

echo $original_string = "ORIGINAL UPPER CASE STRING and Camel Case or mIXed cASe";
echo "<br>";
echo $lowercased_string = strtolower($original_string) . " this has been converted.";
echo "<br>";
echo "The below is now in uppercase.";
echo "<br>";
echo $modified_string = strtoupper($original_string);
echo "<br>";

$var = $original_string . " is the original value.";
echo $var;

Note: Echoing an assigned variable is perfectly valid. You can remove those as needed.

The above will print:

ORIGINAL UPPER CASE STRING and Camel Case or mIXed cASe
original upper case string and camel case or mixed case this has been converted.
The below is now in uppercase.
ORIGINAL UPPER CASE STRING AND CAMEL CASE OR MIXED CASE
ORIGINAL UPPER CASE STRING and Camel Case or mIXed cASe is the original value.

If you want to change the case of the entire string, try: strtoupper( $string ) or strtolower( $string ). If you want to change just the case of the first letter of the string, try: ucfirst( $string) or lcfirst( $string ).

There is also str_replace(), which is case-sensitive. You could do something like str_replace( 'a', 'A', $string ); to replace all lowercase letter 'a' with uppercase letter 'A'.

You may want to have a look at a list of the php string functions.