Gives error on PHP 7 when converting docx to PDF

[successivesoftware]

Here is the message for your reference
FatalErrorException in String.php line 16:
Cannot use 'String' as class name as it is reserved
in String.php line 16
vendor/gears/string/src/String.php

[netizenking]

my composer.json looks like this:

"require":
{
"gears/string": "0.6.0",
"gears/di": "",
"symfony/process": "2.",
"symfony/filesystem": "2.",
"google/apiclient": "1.",
"jakoch/phantomjs-installer": "1.9.8"
}
And i have this statement in my controller method
$pdf = Gears\Pdf::convert("C:\xampp\htdocs\YobisoftUniserve\BEBC00022017.docx");
I am getting this error:

A PHP Error was encountered

Severity: 8192

Message: Non-static method Gears\String\Str::contains() should not be called statically

Anybody with a fix to this error please? Thank you