To install a special extension I had to lower my PHP version. I don't know if this can cause trouble for my other extensions, is there a fast way to check all extensions?
No. Except if you live in a parallel universe where all the extensions are shipped with unit tests. Now you have to test everything manually.
Besides that: Downgrading your PHP version is a terrible idea, especially if it's a version that doesn't get security support anymore (currently that's everything up to 5.4). I'd rather ditch the "special extension", its quality can't be that good if it's not working with a current 5.x version.
If you really can't do without it, patch it to make it compatible.