What is PHP cURL and libcurl version relationship?

I'm running php with curl support on a mac. cURL in php works fine(ish) but I noticed an oddity reported by phpinfo(). It states that curl support is 7.19.7

But using the otool command, I get this:

# otool -L `which php`
   /usr/local/bin/php:
        ...
        /usr/lib/libcurl.4.dylib (compatibility version 6.0.0, current version 6.1.0)
        ...

I was wondering what is the relationship between libcurl's 'current version 6.1.0' and php curl's 'curl support 7.19.7'?

Basically just making sure I don't have a version mismatch, because I'm experiencing some PHP DNS issues that I'm trying to debug.