Here is a php array of all search user agents (Crawlerlist) listed on www.useragentstring.com

public $botAgents = array(
	'008', 'ABACHOBot', 'Accoona-AI-Agent', 'AddSugarSpiderBot', 'AnyApexBot', 'Arachmo', 'B-l-i-t-z-B-O-T', 'Baiduspider', 'BecomeBot', 'BeslistBot', 'BillyBobBot', 'Bimbot', 'Bingbot',
	'BlitzBOT', 'boitho.com-dc', 'boitho.com-robot', 'btbot', 'CatchBot', 'Cerberian Drtrs', 'Charlotte', 'ConveraCrawler', 'cosmos', 'Covario IDS', 'DataparkSearch', 'DiamondBot', 'Discobot',
	'Dotbot', 'EARTHCOM.info', 'EmeraldShield.com WebBot', 'envolk[ITS]spider', 'EsperanzaBot', 'Exabot', 'FAST Enterprise Crawler', 'FAST-WebCrawler', 'FDSE robot', 'FindLinks',
	'FurlBot', 'FyberSpider', 'g2crawler', 'Gaisbot', 'GalaxyBot', 'genieBot', 'Gigabot', 'Girafabot', 'Googlebot', 'Googlebot-Image', 'GurujiBot', 'HappyFunBot', 'hl_ftien_spider', 'Holmes',
	'htdig', 'iaskspider', 'ia_archiver', 'iCCrawler', 'ichiro', 'igdeSpyder', 'IRLbot', 'IssueCrawler', 'Jaxified Bot', 'Jyxobot', 'KoepaBot', 'L.webis', 'LapozzBot', 'Larbin', 'LDSpider',
	'LexxeBot', 'Linguee Bot', 'LinkWalker', 'lmspider', 'lwp-trivial', 'mabontland', 'magpie-crawler', 'Mediapartners-Google', 'MJ12bot', 'MLBot', 'Mnogosearch', 'mogimogi', 'MojeekBot',
	'Moreoverbot', 'Morning Paper', 'msnbot', 'MSRBot', 'MVAClient', 'mxbot', 'NetResearchServer', 'NetSeer Crawler', 'NewsGator', 'NG-Search', 'nicebot', 'noxtrumbot', 'Nusearch Spider',
	'NutchCVS', 'Nymesis', 'obot', 'oegp', 'omgilibot', 'OmniExplorer_Bot', 'OOZBOT', 'Orbiter', 'PageBitesHyperBot', 'Peew', 'polybot', 'Pompos', 'PostPost', 'Psbot', 'PycURL', 'Qseero',
	'Radian6', 'RAMPyBot', 'RufusBot', 'SandCrawler', 'SBIder', 'ScoutJet', 'Scrubby', 'SearchSight', 'Seekbot', 'semanticdiscovery', 'Sensis Web Crawler', 'SEOChat::Bot', 'SeznamBot',
	'Shim-Crawler', 'ShopWiki', 'Shoula robot', 'silk', 'Sitebot', 'Snappy', 'sogou spider', 'Sosospider', 'Speedy Spider', 'Sqworm', 'StackRambler', 'suggybot', 'SurveyBot', 'SynooBot',
	'Teoma', 'TerrawizBot', 'TheSuBot', 'Thumbnail.CZ robot', 'TinEye', 'truwoGPS', 'TurnitinBot', 'TweetedTimes Bot', 'TwengaBot', 'updated', 'Urlfilebot', 'Vagabondo', 'VoilaBot', 'Vortex',
	'voyager', 'VYU2', 'webcollage', 'Websquash.com', 'wf84', 'WoFindeIch Robot', 'WomlpeFactory', 'Xaldon_WebSpider', 'yacy', 'Yahoo! Slurp', 'Yahoo! Slurp China', 'YahooSeeker',
	'YahooSeeker-Testing', 'YandexBot', 'YandexImages', 'YandexMetrika', 'Yasaklibot', 'Yeti', 'YodaoBot', 'yoogliFetchAgent', 'YoudaoBot', 'Zao', 'Zealbot', 'zspider', 'ZyBorg'
);

And this is one way to test if the current request is beeing made by a robot:

foreach($this->botAgents as $agent) {
	if (strstr($_SERVER['HTTP_USER_AGENT'], $agent )) {
		//We have a BOT
	}
}