Update the value of a constant

You can’t alter a constant once it is defined. That is how PHP works. Don’t fight it.

The good news is that you should not be using a constant at all. Use options.

// get your value
// the second parameter is the default
$enable_paypal = get_option('enable_paypal',true);

// set your value based on, I assume, a form of some kind
update_option('enable_paypal',false);

Reference

http://codex.wordpress.org/Function_Reference/get_option
http://codex.wordpress.org/Function_Reference/update_option

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