Imrpove array syntax and visible method name

[peter279k]

Changed log

• Add php-7.2,php-7.3 and php-7.4 tests during Travis CI build.
• It's time to use the short array syntax to replace array syntax because this package requires php-5.6 at least.
• Remove additional white spaces on all PHP codes.
• Add visible syntax (public/protected/private) to let developers know the method is visible/invisible.