parse error syntax error unexpected ' ' expecting variable (t_variable) in my laravel 5.5

"I am using laravel 5.5 version and my php version is 7.0 when my project setup on ubuntu server and open my project on browser showing these error Symfony \ Component \ Debug \ Exception \ FatalThrowableError (E_PARSE) Parse error: syntax error, unexpected '?', expecting variable (T_VARIABLE)"

Symfony \ Component \ Debug \ Exception \ FatalThrowableError (E_PARSE) Parse error: syntax error, unexpected '?', expecting variable (T_VARIABLE)

via Chebli Mohamed