?????????????PHP??????????????????????????????ж?PHP????????5????????????????????????ο?????
????1. isset??????ж?????????????
??????????????????ж???????????????????????ж??????????????????
????????????isset???ж?????????????????????????Ч???array_key_exists??4??????
????<?php
????$a = '';
????$a['c'] = '';
????if (!isset($a)) echo '$a δ???????' . "";
????if (!isset($b)) echo '$b δ???????' . "";
????if (isset($a['c'])) echo '$a ??????????' . "";
????// ???????
????// $b δ???????
????// $a ??????????
????2. empty???????????????”??”
??????????κ????δ?????????????? 0 ?? false ?? ???????”” ?? null????????????顢????κ??????????????ж??empty==true
???????1??δ??????????????empty????”??”
???????2??empty?????????????????????
????<?php
????$a = 0;
????$b = '';
????$c = array();
????if (empty($a)) echo '$a ???' . "";
????if (empty($b)) echo '$b ???' . "";
????if (empty($c)) echo '$c ???' . "";
????if (empty($d)) echo '$d ???' . "";
????3. var == null??????ж????????”??”
??????????? 0 ?? false ?? ???????”” ?? null????????????顢?????ж?? null
?????????empty????????????????δ?????? var == null ???????
????<?php
????$a = 0;
????$b = array();
????if ($a == null) echo '$a ???' . "";
????if ($b == null) echo '$b ???' . "";
????if ($c == null) echo '$b ???' . "";
????// ???????
????// $a ???
????// $b ???
????// Undefined variable: c
????4. is_null???????????????”null”
?????????????????????”null”?????????true
???????1??null???????Сд??$a = null; $a = NULL ????κ?????
???????2?????????????”null”???????????true??0???????????false???????鶼????false
???????3??????δ????????????????
????<?php
????$a = null;
????$b = false;
????if (is_null($a)) echo '$a ?NULL' . "";
????if (is_null($b)) echo '$b ?NULL' . "";
????if (is_null($c)) echo '$c ?NULL' . "";
????// ???????
????// $a ?NULL
????// Undefined variable: c
????5. var === null???????????????”null”?????????????????????”null”
?????????????????????”null”??????????????????”null”?????????true
???????1?????ж??”null”?????????is_null?????????
???????2??????δ????????????????
???????
????PHP?У?”NULL” ?? “??” ??2?????
????isset ????????ж??????????????
????empty ??????? “??”??”??”??”0″??”NULL”??”δ?????” ????????ж??TRUE
????is_null ?????? “NULL” ??????ж??TRUE
????var == null ???? “??”??”??”??”0″??”NULL” ????????ж??TRUE
????var === null ?????? “NULL” ??????ж??TRUE
??????????ж????????????????”NULL”????????? is_null?????????”false”??”0″?????????