bin/cake migrations migrateを実行すると以下のエラーが出力される。

Exception: syntax error, unexpected '?', expecting variable (T_VARIABLE) in [/root/my_app/vendor/symfony/console/Output/Output.php, line 40]
2019-07-14 05:11:27 Error: [ParseError] syntax error, unexpected '?', expecting variable (T_VARIABLE) in /root/my_app/vendor/symfony/console/Output/Output.php on li
ne 40
Stack Trace:
#0 /root/my_app/vendor/composer/ClassLoader.php(322): Composer\Autoload\includeFile('/root/my_app/ve...')
#1 [internal function]: Composer\Autoload\ClassLoader->loadClass('Symfony\\Compone...')
#2 /root/my_app/vendor/symfony/console/Output/StreamOutput.php(31): spl_autoload_call('Symfony\\Compone...')
#3 /root/my_app/vendor/composer/ClassLoader.php(444): include('/root/my_app/ve...')
#4 /root/my_app/vendor/composer/ClassLoader.php(322): Composer\Autoload\includeFile('/root/my_app/ve...')
#5 [internal function]: Composer\Autoload\ClassLoader->loadClass('Symfony\\Compone...')
#6 /root/my_app/vendor/symfony/console/Output/ConsoleOutput.php(30): spl_autoload_call('Symfony\\Compone...')
#7 /root/my_app/vendor/composer/ClassLoader.php(444): include('/root/my_app/ve...')
#8 /root/my_app/vendor/composer/ClassLoader.php(322): Composer\Autoload\includeFile('/root/my_app/ve...')
#9 [internal function]: Composer\Autoload\ClassLoader->loadClass('Symfony\\Compone...')
#10 /root/my_app/vendor/cakephp/migrations/src/Shell/MigrationsShell.php(150): spl_autoload_call('Symfony\\Compone...')
#11 /root/my_app/vendor/cakephp/migrations/src/Shell/MigrationsShell.php(108): Migrations\Shell\MigrationsShell->getOutput()
#12 /root/my_app/vendor/cakephp/cakephp/src/Console/Shell.php(532): Migrations\Shell\MigrationsShell->main('migrations', 'migrate')
#13 /root/my_app/vendor/cakephp/migrations/src/Shell/MigrationsShell.php(164): Cake\Console\Shell->runCommand(Array, true, Array)
#14 /root/my_app/vendor/cakephp/cakephp/src/Console/CommandRunner.php(384): Migrations\Shell\MigrationsShell->runCommand(Array, true)
#15 /root/my_app/vendor/cakephp/cakephp/src/Console/CommandRunner.php(162): Cake\Console\CommandRunner->runShell(Object(Migrations\Shell\MigrationsShell), Array)
#16 /root/my_app/bin/cake.php(12): Cake\Console\CommandRunner->run(Array)
#17 {main}