Add support for v7.3

[abolfazl-moeini]

Hello repository maintainer!

PHP 7.3 is going to release soon and Release Candidate versions are now available on php.net,
How can we switch to this version?

Thanks indeed

[cjwilburn]

It might make sense to wait on this until PHP 7.3 is out of RC and actually released.

[abolfazl-moeini]

Thanks, but this tool is for developing stuff and I think it should provide RC, beta, alpha version.

Please think about that.
Regards

[ikari7789]

I’d be more than happy to add support, but this is mainly just a wrapper for existing brew formulas. If brew adds support for installing 7.3, then I can add support here.