isset() 【1】

Returns?TRUE?if?var?exists and has value other than?NULL,?FALSE?otherwise.

输入可以是多个变量，只有所有的变量为真的时候，返回真

empty()【2】

Returns?FALSE?if?var?has a non-empty and non-zero value.

The following things are considered to be empty:

• "" (an empty string)
• 0 (0 as an integer)
• "0" (0 as a string)
• NULL
• FALSE
• array() (an empty array)
• var $var; (a variable declared, but without a value in a class)

输入只能是一个变量

is_null() 【3】

Returns?TRUE?if?var?is?null?,?FALSE?otherwise.

?

?

?? A variable is considered to be?null?if:

• it has been assigned the constant?NULL.