注意这种方法的判断

<?php
header("Content-type: text/html; charset=utf-8");
$a=1;
$b=2;
$c=3;
$d=4;
if($a==1 and $b==1 or $c==3  and $d==6 )
{
    echo "正确";
}else
{
    echo "error";
}