第一种 $ary['a'] 去掉单引号
$ary = array('a'=>1);
$b = 5;

$str = "fdsfdsf$ary[a]";
$str = "fdsfdsf$b";
echo $str;
第二种 用花括号包起来

$ary = array('a'=>1);
$b = 5;

$str = "fdsfdsf{$ary['a']}";
$str = "fdsfdsf$b";
echo $str;