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

Posted by: admin April 23, 2020 Leave a comment

Questions:

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?

How to&Answers:

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

Answer:

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.

Answer:

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 😀

Answer:

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. “

Answer:

This may not be directly what you’re looking for, but I came across this thread, so here is a solution that I used for an issue I was having (based off of @user187291’s answer):

define('app_path', __DIR__ . '/../../');
const APPLICATION_PATH = app_path;
.
.
.
require_once(APPLICATION_PATH . "some_directory/some_file.php");
.
.
.

Seems to work great!