store constant variable on server via .htaccess

To make switching between hosts automatically You could use SetEnvIf :

.htaccess :

SetEnvIf Host host\.com INDEX_URL=http://host.com/index.php
SetEnvIf Host dev\.host\.com INDEX_URL=http://dev.host.com/index_dev.php

PHP :

$indexUrl = isset($_SERVER['INDEX_URL']) ? $_SERVER['INDEX_URL'] : '';

You can save it as environment variable. Put this in .htaccess:

SetEnv INDEX_URL http://example.com/index.php

And get it in PHP:

$indexUrl = getenv('INDEX_URL');

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