PHP의 중괄호 기능에 대해 얼마나 알고 있습니까?

/**
 *php大括号例解
 *site http://bbs.it-home.org
*/
$arr=array(0=>123, 'name'=>'小猪');
foreach($array as $k=>$v){
 echo "select * from blog_blogs where blog_tags like '%{$arr[$k]}%' order by blog_id"; //加一个大括号只是将作为变量的标志符
}
echo '';
foreach($array as $k=>$v){
 echo "select * from blog_blogs where blog_tags like '%{{$arr[$k]}}%' order by blog_id"; //加两个大括号，外层的将作为普通的字符
}
//用大括号来区分变量
//echo "$arr['name']";//用此句会报语法错误
echo "{$arr['name']}";//此句正常，大括号内的字符将作为变量来处理
//$str{4}在字符串的变量的后面跟上{}大括号和中括号一样都是把某个字符串变量当成数组处理
$str = 'abcdefg';
echo $str{4};
?>