Make sure you install libspf2 development package first.

$ git clone https://github.com/ppadron/php-spf
$ cd php-spf
$ phpize
$ ./configure
$ make
$ sudo make install

Load the extension. (check your PHP configuration files path)

$ echo "extension=spf.so" > /etc/php/conf.d/spf.ini

void        Spf::__construct([int $type[, string $domain[, string $spf]]])
SpfResponse Spf::query(string $ip, string $helo, string $sender[, string $recipient])

• Spf::TYPE_DNS_RESOLV - Use DNS resolver for SPF lookups
• Spf::TYPE_DNS_CACHE - Use DNS cache for SPF lookups (default)
• Spf::TYPE_DNS_ZONE - Use DNS zone file for SPF lookups (requires domain and spf parameters)

string  SpfResponse::getResult();
string  SpfResponse::getHeaderComment();
string  SpfResponse::getReceivedSpf();
string  SpfResponse::getReceivedSpfValue();
string  SpfResponse::getExplanation();
string  SpfResponse::getSmtpComment();
boolean SpfResponse::hasErrors();
boolean SpfResponse::hasWarnings();
array   SpfResponse::getErrors();
array   SpfResponse::getWarnings();

• SpfResponse::RESULT_INVALID - Invalid result
• SpfResponse::RESULT_NEUTRAL - Neutral result (neither pass nor fail)
• SpfResponse::RESULT_PASS - SPF check passed
• SpfResponse::RESULT_FAIL - SPF check failed
• SpfResponse::RESULT_SOFTFAIL - SPF check soft-failed
• SpfResponse::RESULT_NONE - No SPF record found
• SpfResponse::RESULT_TEMPERROR - Temporary error occurred
• SpfResponse::RESULT_PERMERROR - Permanent error occurred

• SpfResponse::ERROR_NO_MEMORY - Out of memory error
• SpfResponse::ERROR_NOT_SPF - Record is not an SPF record
• SpfResponse::ERROR_SYNTAX - Syntax error in SPF record
• SpfResponse::ERROR_MOD_W_PREF - Invalid modifier 'w' preference
• SpfResponse::ERROR_INVALID_CHAR - Invalid character in SPF record
• SpfResponse::ERROR_UNKNOWN_MECH - Unknown mechanism
• SpfResponse::ERROR_INVALID_OPT - Invalid option
• SpfResponse::ERROR_INVALID_CIDR - Invalid CIDR notation
• SpfResponse::ERROR_MISSING_OPT - Missing required option
• SpfResponse::ERROR_INTERNAL_ERROR - Internal error
• SpfResponse::ERROR_INVALID_ESC - Invalid escape sequence
• SpfResponse::ERROR_INVALID_VAR - Invalid variable
• SpfResponse::ERROR_BIG_SUBDOM - Subdomain too large
• SpfResponse::ERROR_INVALID_DELIM - Invalid delimiter
• SpfResponse::ERROR_BIG_STRING - String too large
• SpfResponse::ERROR_BIG_MECH - Mechanism too large
• SpfResponse::ERROR_BIG_MOD - Modifier too large
• SpfResponse::ERROR_BIG_DNS - DNS record too large
• SpfResponse::ERROR_INVALID_IP4 - Invalid IPv4 address
• SpfResponse::ERROR_INVALID_IP6 - Invalid IPv6 address
• SpfResponse::ERROR_INVALID_PREFIX - Invalid prefix length
• SpfResponse::ERROR_RESULT_UNKNOWN - Unknown result
• SpfResponse::ERROR_UNINIT_VAR - Uninitialized variable
• SpfResponse::ERROR_MOD_NOT_FOUND - Modifier not found
• SpfResponse::ERROR_NOT_CONFIG - Not configured
• SpfResponse::ERROR_DNS_ERROR - DNS lookup error
• SpfResponse::ERROR_BAD_HOST_IP - Bad host IP address
• SpfResponse::ERROR_BAD_HOST_TLD - Bad host TLD
• SpfResponse::ERROR_MECH_AFTER_ALL - Mechanism found after 'all'
• SpfResponse::ERROR_INCLUDE_RETURNED_NONE - Include returned 'none'
• SpfResponse::ERROR_RECURSIVE - Recursive include detected
• SpfResponse::ERROR_MULTIPLE_RECORDS - Multiple SPF records found