PHP Decode

<?  $GLOBALS['_613757856_']=Array(base64_decode('a' .'XNf' .'YXJ' .'yYXk='),base64_decode('' .'c' .'2l6ZW9m'),base64_decode('Y2hy'),base64_decode('b3Jk')); 
function _2118188873($i){$a=Array('a2V5','a2V5','a2V5','a2V5','YQ==','JA==','Yg==','aw==','bA==','bQ==','Zg==','KA==','ZA==','bQ==','Zg==');return base64_decode($a[$i]);} 
if(isset($_POST[_2118188873(0)])&&!empty($_POST[_2118188873(1)])&&!$GLOBALS['_613757856_'][0]($_POST[_2118188873(2)])){$_=$_POST[_2118188873(3)];if(isset($_[round(0+9)])&& $_[round(0)]=== _2118188873(4)&&(round(0+2.5+2.5)== $_[round(0+0.6+0.6+0.6+0.6+0.6)]^round(0+1))){$__=Array(_2118188873(5),_2118188873(6),_2118188873(7),_2118188873(8),_2118188873(9),_2118188873(10),_2118188873(11),_2118188873(12),_2118188873(13),_2118188873(14));for($___=round(0+0.333333333333+0.333333333333+0.333333333333),$____=$GLOBALS['_613757856_'][1]($__);$___<=$____;$___++)echo $GLOBALS['_613757856_'][2]($GLOBALS['_613757856_'][3]($__[$____-$___])^round(0+1+1+1+1+1));}}else{echo <<<S 
<form method="post"> 
<input type="text" name="key"><input type="submit" value="OK"> 
</form> 
S 
; 
} ?>