(PHP 4, PHP 5, PHP 7)
isset — 检测变量是否已设置并且非 NULL
isset(mixed$var[,mixed$...] ) :bool
如果 var 存在并且值不是 NULL...(isset($a[0])); //true $a[0] = "h"
var_dump(isset($b)); //true
var_dump(isset($c)); //true
var_dump...(isset($d)); //true
var_dump(isset($f)); //false
var_dump(isset($a,$b,$c,$d)); //true
var_dump(...isset($a,$b,$c,$d,$f)); //false
unset($a);
var_dump(isset($a)); //false
(PHP 4, PHP 5, PHP 7)
empty...($array['test'])); //true
var_dump(isset($array['str'])); //false
var_dump(isset($array['child']