Whitelist PHP formulas in shadowed header audit check

Library/Homebrew/extend/os/mac/formula_cellar_checks.rb

@@ -6,6 +6,8 @@ module FormulaCellarChecks
       formula.name.start_with?(formula_name)
     end
 
+    return if formula.name =~ /^php\d+$/
+
     return if MacOS.version < :mavericks && formula.name.start_with?("postgresql")
     return if MacOS.version < :yosemite  && formula.name.start_with?("memcached")