A - Keywords

The following words are reserved by PHP. You can’t use any of them as the name of a variable, function, class, constant, or namespace: the parser has already claimed them for something else.

Control flow

if · else · elseif · endif · while · endwhile · do · for · endfor · foreach · endforeach · as · switch · endswitch · case · default · match · break · continue · goto · return · yield

Class-related

class · interface · trait · enum · extends · implements · new · clone · instanceof · abstract · final · public · protected · private · readonly · static · const · var · function · fn · use

Error handling

try · catch · finally · throw

Namespaces and includes

namespace · use · require · require_once · include · include_once

Other

echo · print · declare · enddeclare · global · list · array · isset · unset · empty · exit · die · and · or · xor · not · int · float · bool · string · null · true · false · void · mixed · never · self · parent

That last group of type names (int, string, null, true, false, and the rest) deserves a note: they only became reserved gradually, as PHP added them as proper type declarations. Older code sometimes used String or Int as class names, back when that was still legal. It isn’t anymore.

use shows up in two groups above because it does two unrelated jobs: importing names from a namespace (Chapter 7) and capturing variables into a closure (Chapter 15). Same word, same reservation, different context.

None of these can be repurposed, no matter how well the name would otherwise fit your code. Try to name a variable $class: that one’s fine, actually. Keywords only block bare identifiers, not variable names after the $. Try to name a function list() or a class Match, and PHP will stop you at parse time, not at runtime. Better there than in production.