In PHP is there a function to do a reverse lookup on a domain name to find out how many websites are hosted on the particular shared hosting server that domain name is hosted on. Or, a way to do this with PHP?
Now, I'm already aware of the online services that offer this. However, I want to write a script to do it myself. I just can't figure it out.
Any suggestions that are not suggesting the use of a 3rd party service would be great.
There is no sure-fire way to do what you are asking.
In DNS, a site's IP address, such as "1.5.7.9", has associated with it a domain name like "9.7.5.1.in-addr.arpa". This reverse name may have PTR records pointing to the domain name. So, "example.com" may map to "1.5.7.9" with an A record, and "9.7.5.1.in-addr.arpa" may point back to "example.com".
An IP address may have more than one PTR record. But, generally, they do not. So the only way to know all the domain names that share one IP is to either search and remember all domain names (not feasible), or to get the information from some other source.
Furthermore, a given computer may have many IPs assigned to it. There's no way to know how many.