Parse error: syntax error, unexpected '?', expecting variable (T_VARIABLE)

My PHP version in server is : PHP 7.2.11 and Laravel version of my application is 5.5.*

While I recently uploaded my application in server, its giving the error message,

Parse error: syntax error, unexpected '?', expecting variable (T_VARIABLE)

in file

vendor/symfony/translation/Translator.php in line 79.

Can anyone please help me on this?