I'm a complete Linux beginner. Thank you.
We are making a system using Laravel.
Install Laravel5.5 on Linux and execute php artisan command
The following error will occur:

[Addition] I checked the following errors and found that Laravel 5.6 is required, but I cannot install Laravel 5.6 for the rental server. In the local environment, it runs on Laravel 5.5. I would be happy if you couldn't understand why the Laravel 5.6 is required. Thank you in advance.

Error message
PHP Parse error: syntax error, unexpected 'function' (T_FUNCTION), expecting identifier (T_STRING) or \\ (T_NS_SEPARATOR) in /home/users/0/mond.jp-funfun/fun-corp/ vendor/myclabs/deep-copy/src/DeepCopy/deep_copy.php on line 5
Applicable source code
<? php
namespace DeepCopy;
use function function_exists;
if (false === function_exists ('DeepCopy \ deep_copy')) {
    / **
     * Deep copies the given value.
     * @param mixed $value
     * @param bool $useCloneMethod
     * @return mixed
     * /
    function deep_copy ($value, $useCloneMethod = false)
        return (new DeepCopy ($useCloneMethod))->copy ($value);
Supplemental information (FW/tool version etc.)

Laravel 5.5 is installed on Lollipop PHP7.1 server. Thank you in advance.