isset();判读值是否值在unset();取消变量<?php$a=array("a","b","c");$b=$a;$b[2]="q";$c=$a;echo $a[2];echo $b[2];echo $c[2];?>结果cqc;$b=&$a;&是引用传值<?php$a=array("a","b","c");$b=&$a;$b[2]="q";$c=$a;echo $a[2];echo $b[2];echo $c[2];?>结果qqq;