check to see if a domain is available or not using PHP?

You can use checkdnsrr or gethostbyname:

Documentation:

http://www.php.net/checkdnsrr

http://www.php.net/gethostbyname

Example checkdnsrr:

<?php
 if ( checkdnsrr('example.com.', 'ANY') ) {
  echo "DNS Record found";
 }
 else {
  echo "NO DNS Record found";
 }
?>

Example gethostbyname:

<?php
 $domain = 'example.com';
 if ( gethostbyname($domain) != $domain ) {
  echo "DNS Record found";
 }
 else {
  echo "NO DNS Record found";
 }
?>

It depends on what you mean by "available." If you mean available for registration, it is not possible to determine based on DNS information alone. The whois system must be used. An easy way to test is to take an unused domain name and set the nameservers to something invalid. DNS will not be available, but the domain is still unavailable for registration. I just tested the suggestions of checkdnsrr(), gethostbyname(), and dns_get_record(). All show that no DNS is returned for a domain that cannot be registered.

The following question offers some more details: Checking if a domain name is registered