PHP 7 is not supported
Currently there is no PHP 7 compatible version of vtCompose. The only supported PHP version is 5.6. Perhaps the most significant reason for the incompatibility is the VTCompose\String namespace and the String class inside this namespace. As of PHP 7 string is a reserved word and cannot be used as a class or namespace name.
The VTCompose\String namespace and the String class inside this namespace should be renamed so that they don't clash with the reserved word string. Any other change necessary to make vtCompose compatible with PHP 7.2 should be done (see @todo DocBlock tags in the code for some of these changes).