?????????????PHP??????????????????????????????卸?PHP????????5????????????????????????慰?????
????1. isset??????卸?????????????
??????????????????卸???????????????????????卸??????????????????
????????????isset???卸?????????????????????????效???array_key_exists??4??????
????<?php
????$a = '';
????$a['c'] = '';
????if (!isset($a)) echo '$a 未???????' . "";
????if (!isset($b)) echo '$b 未???????' . "";
????if (isset($a['c'])) echo '$a ??????????' . "";
????// ???????
????// $b 未???????
????// $a ??????????
????2. empty???????????????”??”
??????????魏????未?????????????? 0 ?? false ?? ???????”” ?? null????????????椤�????魏??????????????卸??empty==true
???????1??未??????????????empty????”??”
???????2??empty?????????????????????
????<?php
????$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??????卸????????”??”
??????????? 0 ?? false ?? ???????”” ?? null????????????椤�?????卸?? null
?????????empty????????????????未?????? var == null ???????
????<?php
????$a = 0;
????$b = array();
????if ($a == null) echo '$a ???' . "";
????if ($b == null) echo '$b ???' . "";
????if ($c == null) echo '$b ???' . "";
????// ???????
????// $a ???
????// $b ???
????// Undefined variable: c
????4. is_null???????????????”null”
?????????????????????”null”?????????true
???????1??null???????小写??$a = null; $a = NULL ????魏?????
???????2?????????????”null”???????????true??0???????????false???????槎�????false
???????3??????未????????????????
????<?php
????$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?????卸??”null”?????????is_null?????????
???????2??????未????????????????
???????
????PHP?校?”NULL” ?? “??” ??2?????睢�
????isset ????????卸??????????????
????empty ??????? “??”??”??”??”0″??”NULL”??”未?????” ????????卸??TRUE
????is_null ?????? “NULL” ??????卸??TRUE
????var == null ???? “??”??”??”??”0″??”NULL” ????????卸??TRUE
????var === null ?????? “NULL” ??????卸??TRUE
??????????卸????????????????”NULL”????????? is_null?????????”false”??”0″?????????