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    <p>If the value of the constant is variable, then it is not a constant, but a variable. </p> <p>Since you are (correctly) trying to keep your application stuff capsuled and away from the global scope, you might be interested in the Registry Pattern. A Registry is basically a glorified array that stores whatever you throw into it and is globally accessible throughout your application.</p> <p>Some resources:</p> <ul> <li><a href="http://www.patternsforphp.org/index.php?title=Registry" rel="nofollow noreferrer">http://www.patternsforphp.org/index.php?title=Registry</a></li> <li><a href="http://www.brandonsavage.net/the-registry-pattern-reexamined/" rel="nofollow noreferrer">http://www.brandonsavage.net/the-registry-pattern-reexamined/</a></li> <li><a href="http://blog.fedecarg.com/2007/10/26/registry-pattern-or-dependency-injection-container/" rel="nofollow noreferrer">http://blog.fedecarg.com/2007/10/26/registry-pattern-or-dependency-injection-container/</a></li> </ul> <hr> <p><strong>EDIT</strong> If you are truly desperate and <em>have to have</em> the constant redefined, you can use </p> <ul> <li><a href="http://de.php.net/manual/en/function.runkit-constant-redefine.php" rel="nofollow noreferrer"><code>runkit_constant_redefine</code></a> — Redefine an already defined constant</li> </ul> <p>Runkit might not be available if you are on shared hosting and I consider needing it a code smell, but here is how you'd basically do it (in your bootstrap)</p> <pre><code>if ( file_exists('xyc') ) { runkit_constant_redefine('Constants::USERNAME', 'xyz'); } </code></pre> <hr> <p><strong>EDIT</strong> Some more options (all of which not exactly pretty either):</p> <pre><code>class Abc { const FOO = 1; const BAR = 2; } class Xyz extends Abc { const FOO = 2; } class_alias(file_exists('abc') ? 'Abc' : 'Xyz', 'Constants'); </code></pre> <p>For this you would rename your current constants class to Abc and add a second class Xyz to extend it and overwrite the USERNAME constant (FOO in the example). This would obviously break your code, because you used to do <code>Constants::USERNAME</code>, so you have to create an alias for the former class name. Which class Constants will point to, is decided with the conditional check. This requires PHP5.3.</p> <p>A pre-5.3 solution would be to simply save the Constants class file under two different names, e.g. abc_constants.php and xyz_constants.php, modify the latter accordingly to hold USERNAME <em>xyz</em> and then <code>include</code> either or depending on the file check.</p> <p>Or replace the value of USERNAME with a placeholder and instead of including the class you load it into a variable as a string. Then you replace the placeholder according to the filecheck result and <code>eval</code> the string, effectively including the class this way.</p> <p><strong>But I have to say it again: I strongly suggest refactoring your code over using these.</strong></p>
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    1. COHe didn't say it was variable, he said it was conditional, which is why the define function exists.
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    2. CO@NSD the OP explicitly states *for lets say USERNAME constant, value can be either xyz or abc*. how is that not variable?
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    3. COAs is usually the case on SO, the question was edited by someone who didn't pay a lot of attention to it. What he actually said was that it was conditionally defined based on whether or not a file existed. Since it's unlikely that this file is going to be created and deleted repeatedly at runtime, this is overkill unless you just want to impress your boss with your line count at the end of the day.
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