问一个Js的语法,在PHP该怎样表示?解决方法

问一个Js的语法,在PHP该怎样表示?
Javascript:

JScript code

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var a = 'a';
var b = null;
alert(a||b);
//这会输出a的变量值,,想问一下php应该怎么表示?

PHP code

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$a = 'A';
$b = null;
echo $a || $b;
//这样只会输出1或0,,
//我以前记得在哪个Oauth的开发文档上看过...但我现在忘记了...
//不知道有能人能告诉一下本人..Thanks

------解决方案--------------------
$a = 'A';
$b = null;
$c = $a or $c = $b;
echo $c;