较长数字 字符串 的比较有关问题

<br />
$a = "222111565652521142";<br />
		$b = "222111565652521139";<br />
		$c = 222111565652521142;<br />
		$d = 222111565652521139;<br />
<br />
		var_dump($a);<br />
		echo "<br/>";<br />
		var_dump($b);<br />
		echo "<br/>";<br />
		var_dump($c);<br />
		echo "<br/>";<br />
		var_dump($d);<br />
		echo "<br/>";<br />
<br />
		if ($a == $b) {<br />
			echo 3;<br />
		}else{<br />
			echo 31;<br />
		}<br />
		echo "<br/>";<br />
<br />
		if ($c == $d) {<br />
			echo 4;<br />
		}else{<br />
			echo 41;<br />
		}<br />
		echo "<br/>";<br />

<br />
<br />
<br />
string '222111565652521142' (length=18)<br />
<br />
<br />
string '222111565652521139' (length=18)<br />
<br />
<br />
float 2.2211156565252E+17<br />
<br />
<br />
float 2.2211156565252E+17<br />
<br />
<br />
<br />
31<br />
4<br />