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Самый простой способ -

define('MY_CONSTANT', 42);

$my_constant = MY_CONSTANT;
echo "This is my constant: $my_constant";

Другой способ использования (s)printf

define('MY_CONSTANT', 42);

// Note that %d is for numeric values. Use %s when constant is a string    
printf('This is my constant: %d', MY_CONSTANT);

// Or if you want to use the string.

$my_string = sprintf('This is my constant: %d', MY_CONSTANT);
echo $my_string;