How to know which version of the Internet Protocol (IP) a client is using when connecting to my server?

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Check for IPv4

$ip = "255.255.255.255";
if(filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_IPV4)) {    
    echo "Valid IPv4";
}
else {
    echo "Invalid IPv4";
}

Check for IPv6

$ip = "FE80:0000:0000:0000:0202:B3FF:FE1E:8329";    
if(filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_IPV6)) {
    echo "Valid IPv6";
}
else {
    echo "Invalid IPv6";
}

For more, check PHP function filter_vars and list of filters for validation.

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