关键字是编程语言里事先定义好并赋予特殊含义的单词，也称为保留字。和其他语言一样，PHP保留了许多关键字。
| and | or | xor | __FILE__ | exception |
| __LINE__ | array() | as | break | case |
| class | const | continue | declare | default |
| die() | do | echo | else | elseif |
| empty() | enddeclare | endfor | endforeach | endif |
| endswitch | endwhile | eval() | exit() | extends |
| for | foreach | function | global | if |
| include | include_once | isset() | list() | new |
| require | require_once | return | static | |
| switch | unset() | use | var | while |
| __FUNCTION__ | __CLASS__ | __METHOD__ | final | php_user_filter |
| interface | implements | extends | public | private |
| protected | abstract | clone | try | catch |
| throw | this |
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