PHP Check if Hostname resolved to local machine - php

Question

I want to use PHP to check if a specified host name is resolving to the server on which the PHP script is running. By host name, I'm mean the same as a domain name in my particular context.
I say for example, let's say I have a server and it's public IP address is 123.123.123.123 or something similar. My DNS server is resolving two domains to this IP address: domain1.com and domain2.com . I'm accessing the PHP page on the server by domain1.com but I want the script to tell me if domain2.com is resolving to the server and return an appropriate result, like echoing 'yes' or 'no'.
I'm not sure how to approach this. I'm thinking of two possibilities:
Try something similar to a 'ping' of the hostname and see if there's a response, though I don't think this will work because it's local to local and may require the hostname to be resolved in the server's hosts file, which defeats the point of this (I want to see if the hostname is resolving for other clients on the Internet, not the local machine), or
check the hostname's DNS server to see if it resolves to the server's public IP, in which case I'd have to get the public IP somehow in PHP.
How would I approach this?
An example PHP script would be:
function host_resolves($host){
// get server public IP
// check if hostname is a valid domain name
// try to connect to the hostname's DNS server
// query the DNS server and compare IP (maybe in A record?) to public IP
// split off a sub-domain and check for a CNAME if needed
}
if(host_resolves('domain2.com')){
echo 'yep!';
}
else{
echo 'nup :(';
}