Can I use string concatenation to define a class CONST in PHP?

Question

I know that you can create global constants in terms of each other using string concatenation:

define('FOO', 'foo');
define('BAR', FOO.'bar');  
echo BAR;

will print 'foobar'.

However, I'm getting an error trying to do the same using class constants.

class foobar {
  const foo = 'foo';
  const foo2 = self::foo;
  const bar = self::foo.'bar';
}

foo2 is defined without issue, but declaring const bar will error out

Parse error: syntax error, unexpected '.', expecting ',' or ';'

I've also tried using functions like sprintf() but it doesn't like the left paren any more than the string concatenator '.'.

So is there any way to create class constants in terms of each other in anything more than a trivial set case like foo2?

Solution 1

Imho, this question deserves an answer for PHP 5.6+, thanks to @jammin comment

Since PHP 5.6 you are allowed to define a static scalar expressions for a constant:

class Foo { 
  const BAR = "baz";
  const HAZ = self::BAR . " boo\n"; 
}

Although it is not part of the question, one should be aware of the limits of the implementation. The following won't work, although it is static content (but might be manipulated at runtime):

class Foo { 
  public static $bar = "baz";
  const HAZ = self::$bar . " boo\n"; 
}
// PHP Parse error:  syntax error, unexpected '$bar' (T_VARIABLE), expecting identifier (T_STRING) or class (T_CLASS)

class Foo { 
  public static function bar () { return "baz";}
  const HAZ = self::bar() . " boo\n"; 
}
// PHP Parse error:  syntax error, unexpected '(', expecting ',' or ';'

For further information take a look at: https://wiki.php.net/rfc/const_scalar_exprs and http://php.net/manual/en/language.oop5.constants.php

Solution 2

The only way is to define() an expression and then use that constant in the class

define('foobar', 'foo' . 'bar');

class Foo
{
    const blah = foobar;
}

echo Foo::blah;

Another option is to go to bugs.php.net and kindly ask them to fix this.

Solution 3

Always fall back to the trusty manual for stuff like this.

Regarding constants:

The value must be a constant expression, not (for example) a variable, a property, a result of a mathematical operation, or a function call.

So... "no" would be the answer :D

Solution 4

For class constants, you can't assign anything other than a constant expression. Quoting the PHP manual:

"The value must be a constant expression, not (for example) a variable, a property, a result of a mathematical operation, or a function call. "

View more solutions

Comments

[selfsimilar]

I don't understand the rationale for different setter behavior between global constants and class constants. I think I will visit bugs.php.net shortly :) But for the short-term, I'll probably just move the constants out of the class and then copy them into the class. So kludgy!

[selfsimilar]

bug.php.net basically told me they wouldn't fix it. "This is a limitation in the implementation. For the class constant we need a constant value at compile time and can't evaluate expressions. define() is a regular function, evaluated at run time and can therefore contain any value of any form. changing this would mean to add an execution phase in the compiler ..." Which is actually BS, as the following code works: define( 'BAR', ThisWorks::foo . 'bar' ); class ThisWorks { const foo = 'foo'; const bar = BAR; } echo ThisWorks::bar; this will output 'foobar' without an error.

[Matthew]

For anyone else looking for it, the bug is #42355.

[Eric G]

Chock another one up for PHP: A Fractal of Bad Design

[helvete]

@StefanNch: There is no need for updating the question for PHP 5.6 since the problem OP had simply don't exist any longer.

[StefanNch]

@helvete this problem can occur (for others programmers, not just OP) because at the moment (end of 2015) php.net still provides binaries for 5.4 and 5.5 windows version! Also for mint/ubuntu (probably debian also) the php out of the box version is 5.5!

[helvete]

@StefanNch: My reaction regarded only PHP 5.6; Of course the problem exists on lower versions - that's out of debate..