[wp-trac] [WordPress Trac] #56779: wp_mail header ignores multiple headers with same name

[WordPress Trac]

#56779: wp_mail header ignores multiple headers with same name
----------------------------+-----------------------------
 Reporter:  pentatonicfunk  |      Owner:  (none)
     Type:  enhancement     |     Status:  new
 Priority:  normal          |  Milestone:  Awaiting Review
Component:  Mail            |    Version:
 Severity:  normal          |   Keywords:
  Focuses:                  |
----------------------------+-----------------------------
 Context:
 https://github.com/WordPress/wordpress-develop/blob/trunk/src/wp-
 includes/pluggable.php#L344
 Snippet `wp_mail`

 {{{#!php
 default:
     // Add it to our grand headers array.
     $headers[ trim( $name ) ] = trim( $content );
     break;
 }}}
 Passing headers this way:
 {{{#!php
 $headers[] = 'x-my-things: thing1';
 $headers[] = 'x-my-things: thing2';
 }}}
 Will resulting headers only generated one ( the last one )
 {{{#!php
 $headers['x-my-things'] = 'thing2';
 }}}

 Expectation, it should retain the headers index, should behave similarly
 with `phpmaailer::addCustomHeader`, headers indexed by numeric, and each
 item contains name and value.

 Example use case Mailgun `x-mailgun-tag`,
 https://documentation.mailgun.com/en/latest/user_manual.html?highlight=x
 -mailgun-tag

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Ticket URL: <https://core.trac.wordpress.org/ticket/56779>
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