http://qiita.com/kirksencho/items/81c499ed159cf4b0a9e9

GetZoneInfo.php

<?php

class GetZoneInfo {

    const ORIGIN_SIGNATURE = '$ORIGIN';
    const IN_SIGNATURE     = 'IN';
    const A_RECORD         = "A";
    const CNAME_RECORD     = "CNAME";

    protected $fqdnList = [];
    protected $cnames   = [];

    public function __construct($dirPath) {
        $this->parse($dirPath);
    }

    public function getFQDN($ipAddress) {
        $ret = [];

        foreach ( $this->fqdnList as $fqdn ) {
            if ( $fqdn->ipAddress !== $ipAddress ) {
                continue;
            }
            $ret[] = $fqdn->fqdn;
        }

        return $ret;
    }

    protected function parse($dirPath) {
        $iter = new RecursiveIteratorIterator(
            new RecursiveDirectoryIterator($dirPath)
        );
        // Initialize first
        $isFirstOrigin = true;

        // Get file path from file list.
        foreach ( $iter as $file ) {
            if ( $file->isDir() ) {
                continue;
            }
            $lines = file($file->getPathname());
            foreach ( $lines as $line ) {
                $line = preg_replace("/[　\s]+/", " ", trim($line));
                $this->parseLineData(explode(" ", $line), $isFirstOrigin);
            }
        }

        // Parse stacked cnames and convert to FQDN list
        $this->mergeCnamesToFQDN();
    }

    protected function parseLineData(array $lineData, &$isFirstOrigin) {
        switch ( $lineData[0] ) {
        case static::ORIGIN_SIGNATURE:
            if ( $isFirstOrigin === true ) {
                $isFirstOrigin = false;
            } else {
                $this->domain = $lineData[1];
            }
            break;
        case static::A_RECORD:
            $info =           new stdClass;
            $info->fqdn      = $this->domain;
            $info->ipAddress = $lineData[1];

            $this->fqdnList[] = $info;
            break;
        default:
            $this->detectLineData($lineData);
            break;
        }
    }

    private function detectLineData(&$lineData) {
        if ( ! isset($lineData[1]) ) {
            return;
        }

        switch ( $lineData[1] ) {
        case static::IN_SIGNATURE:
            $this->domain = $lineData[0];
            break;
        case static::A_RECORD:
            $info            = new stdClass;
            $info->fqdn      = $lineData[0] . "." . substr($this->domain, 0 , -1);
            $info->ipAddress = $lineData[2];

            $this->fqdnList[] = $info;
            break;
        case static::CNAME_RECORD:
            $cname            = new stdClass;
            $cname->origin    = substr($this->domain, 0, -1);
            $cname->fqdn      = $lineData[0] . "." . $cname->origin;
            $cname->ipAddress = $lineData[2];

            $this->cnames[] = $cname;
            break;
        }
    }

    protected function mergeCnamesToFQDN() {
        foreach ( $this->cnames as $ci => $cname ) {
            if ( false === ($fqdn = $this->findByFQDNList($cname)) ) {
                continue;
            }
            $cname->ipAddress = $fqdn->ipAddress;
            $this->fqdnList[] = $cname;
        }
    }

    protected function findByFQDNList(stdClass $cname) {
        foreach ( $this->fqdnList as $fqdn ) {
            if ( $cname->ipAddress . "." . $cname->origin === $fqdn->fqdn ) {
                return $fqdn;
            }
        }

        return false;
    }
}