Description:
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Trying to use preg_replace to change text to uppercase in-between <uc><uc>. However expected result doesn't occur and the text remains lowercase. No errors are logged. However adding some text after $1 outputs this text in uppercase, but the text in-between <uc><uc> remains as lowercase.

Reproduce code:
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<?
# Test 1: Attempt to replace 'my' with 'MY'
$text	=	'this is <uc>my<uc> string';
$text	=	preg_replace('/\<uc\>(.*)\<uc\>/', strtoupper('$1'), $text);
echo $text.'<br><br>';

# Test 2: Add something after $1 which produces expected output
$text	=	'this is <uc>my<uc> string';
$text	=	preg_replace('/\<uc\>(.*)\<uc\>/', strtoupper('$1 cool'), $text);
echo $text;
?>

Expected result:
----------------
# Expected result for test 1
this is MY string

# Expected result for test 2
this is MY COOL string

Actual result:
--------------
# Actual result for test 1
this is my string

# Actual result for test 2
this is my COOL string