问题:

For example, is the following program meaningful, and if so what should it print?

<?php
FuncTIon fOo($x) { eChO $x; }
FOO('bar');
IF (TRuE) { echO 'qux'; }
?>

My interpreter runs it and prints barqux, implying the keywords are not case-sensitive:

$ php case_sensitive_keywords.php 
barqux
$ php --version
PHP 5.5.7-1+sury.org~precise+1 (cli) (built: Dec 12 2013 21:37:40)

However, this same question was asked last year, and the answers say that keywords are case-sensitive, in direct contradiction to what my PHP interpreter appears to tell me!

回答1:

Case sensitive (both user defined and PHP defined)

variables
constants
array keys
class properties
class constants

Case insensitive (both user defined and PHP defined)

functions
class constructors
class methods
keywords and constructs (if, else, null, foreach, echo etc.)

回答2:

No. Keywords are case-insensitive. Lerdorf et al., Programming PHP, page 17:

The names of user-defined classes and functions, as well as built-in constructs and keywords such as echo, while, class, etc., are case-insensitive. Thus, these three lines are equivalent:
echo("hello, world");
ECHO("hello, world");
EcHo("hello, world");