A keyword (or reserved word) is a word set aside by the language for its core functionality—you cannot give a variable, function, class, or constant the same name as a keyword
Table to Lists the keywords in PHP, which are case-insensitive
| _CLASS_ | echo | insteadof |
| _DIR_ | else | interface |
| _FILE_ | elseif | isset() |
| _FUNCTION_ | empty() | list() |
| _LINE_ | enddeclare | namespace |
| _METHOD_ | endfor | new |
| _NAMESPACE_ | endforeach | or |
| _TRAIT_ | endif | |
| _halt_compiler() | endswitch | private |
| abstract | endwhile | protected |
| and | eval() | public |
| array() | exit() | require |
| as | extends | require_once |
| break | final | return |
| callable | for | static |
| case | foreach | switch |
| catch | function | throw |
| class | global | trait |
| clone | goto | try |
| const | if | unset() |
| continue | implements | use |
| declare | include | var |
| default | include_once | while |
| die() | instanceof | xor |
| do |