How can I extract strings from one file to insert (modified) in a different file?

problem is the following: I have an xml file with data and I am looking for a small part of the data to write it into a new file: content has been shortened by request: snippet if type=dhcp-client:

yes
          
        
        Firewall

snippet if type=static

Firewall
        
          
        
        192.168.0.2
        255.255.255.0
        192.168.0.1
      
    
      
        
          
            
              
                
              
            
          
        
      
      
        
          
            
              
                
                  
                    192.168.0.1
                  
                  ethernet1/4
                  0.0.0.0/0

the four relevant values are unique (or nonexistent) within the "system" tag `` things like ip-address might appear again elsewhere outside of `` but i am only checking for the ones inside system, if the type is not static dont appear, i set it to dhcp-client this is what I need as a result in a file if the type is dhcp:

type=dhcp-client

this is what I need as a result in a file if the type was static:

type=static
ip-address=192.168.0.2
default-gateway=192.168.0.1
netmask=255.255.255.0

I am not sure how to accomplish this efficiently and integrated inside an existing **PHP** file (so either work with exec or better yet use php only). I am also limited to tools that are installed by default on an ubuntu server system and would be unable to use other packages. PS: this is actually the whole/complete use-case, I will not need to produce other output other than these two examples. Thanks for any help or pointers :)