Description:
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Using the code sample below, you could see that it adds a "\r\n" to the end of the mail headers irrelevant to $additional_headers containing a meaningful value.
Even using an
empty($addParams) ? NULL/false : implode()
replacement, it does not fix the problem. The resulting message will still start from second line. Which is bad for look.
To beat it, you're forced to check for empty($addParams) before invoking mail() (and to have two separate mail() lines in your code, which does not lead to the code clarity)...

Test script:
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<?php

$addParams = array();
mail('someuser@example.com', 'testsubj', 'Body part', implode("\r\n", $addParams));