请问 !empty($a) ? $a : $b 和 $a ?: $b 有什么区别

!empty($arr['key1']) 会先判断 isset()，如果 !$arr['key1'] 的话会尝试访问不存在的变量，产生警告，如下语句可以证明：

# 不会警告
$ php -r "!empty($arr['key1']);"
# 警告
$ php -r "!$arr['key1'];"                                           
PHP Notice:  Undefined variable: arr in Command line code on line 1 
PHP Stack trace:                                                    
PHP   1. {main}() Command line code:0                               

Notice: Undefined variable: arr in Command line code on line 1      

Call Stack:                                                         
    0.0005     366416   1. {main}() Command line code:0             
# 同警告
$ php -r "$arr['key1']?:'111';"
PHP Notice:  Undefined variable: arr in Command line code on line 1
PHP Stack trace:
PHP   1. {main}() Command line code:0

Notice: Undefined variable: arr in Command line code on line 1

Call Stack:
    0.0003     366536   1. {main}() Command line code:0