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checkdnsrr

This simply tells us if a particular DNS record was found, so could be useful to see if an entered hostname was valid within an email check.

checkdnsrr -- Check DNS records corresponding to a given Internet host name or IP address
$check=checkdnsrr("hostname", "A");

Let us assume a couple of entries, the first being the valid smirkingloon.com, the second being the invalid liamdelahuntyy.com (extra y)

Results:

$check=checkdnsrr("smirkingloon.com", "A");

Check: 1

$check=checkdnsrr("liamdelahuntyy.com", "A");

Check: 1

Notice the later domain fails and doesn't return a value

See also gethostbyaddr, and Ban Access from an IP address with PHP

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