SQL Query Builder in PHP
Layout for code:
Look at PSR spec: http://www.php-fig.org/psr/psr-2/

for comments:
https://www.phpdoc.org/docs/latest/getting-started/your-first-set-of-documentation.html#writing-a-docblock

Finally is your code testable and possibility to extend without modifying it?

Also "(static) $instance" doesn't feel right to me for some reason, try different name?