PHP Parse error: syntax error, unexpected ':' in /.../public_html/concrete/bootstrap/configure.php on line 21

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Concrete vers 8.2.1
PHP 5.6

Line 21 reads:
*/ @date_default_timezone_set(@date_default_timezone_get() ?: 'UTC'); /* *


I looked in PHP docs and I can't figure out what is wrong. I'm not well-versed in coding and usually C-5 just works. Sure could use some direction, please

bluPaula