Etant donné que la syntaxe de la fonction pour envoyer un courriel en php est

  bool mail ( string $to , string $subject , string $message 
  [, string $additional_headers 
    [, string $additional_parameters ]
  ] 
  )

et la solution pour les headers http est de la forme

  header("content-type: text/plain; charset=utf-8")
  

La suggestion suivante me paraît sensée:

Conseil de php.net pour les caractères à accent, etc.

Zane @ MLI 08-Sep-2010 10:17
Italian users cursing against "È" and other uppercase-accented-vowels ("vocali maiuscole accentate"") in subjects! While the lowercase ones ("è", "é" and so on) work as expected, qmail doesn't handle the uppercase ones.

To fix it, the only way I found was this:

  <?php
   
  function mail_utf8($to, $subject = '(No subject)', $message = '', $header = '') {
    $header_ = 'MIME-Version: 1.0' . "\r\n" . 'Content-type: text/plain; charset=UTF-8' . "\r\n";
    mail($to, '=?UTF-8?B?'.base64_encode($subject).'?=', $message, $header_ . $header);
  }
  
  ?>

It should apply to other languages too.