Thank you for your fast reply, the code is below:
<?php #the HTML form that can be written dynamically
$form ="<form action=\"$PHP_SELF\" method=\"post\">";
$form.="<br>Nom: <input type=\"text\" name=\"username\"";
$form.=" size=\"30\" value=\"$username\" maxlength=\"35\" > <br> <br>";
$form.="Email: <input type=\"text\" name=\"useraddr\"";
$form.=" size=\"30\" value=\"$useraddr\" maxlength=\"35\"> <br> <br>";
$form.="Message:<br><textarea name=\"comments\" ";
$form.="cols=\"40\" rows=\"10\">$comments</textarea>";
$form.="<br><br> <input type=\"submit\" name=\"sent\" ";
$form.="value=\"Envoyez votre Message\"></form>";
#execute this code if the form has been submitted once
if($sent)
{ $valid=true; #set variable default value
#check username field is not blank
if( !$username )
{ $errmsg.="Vous avez oublié d'écrire votre nom...<br>"; $valid = false; }
#check email useraddr field is not blank
if( !$useraddr )
{ $errmsg .="Vous avez oublié d'écrire votre adresse email...<br>"; $valid = false; }
else
{ $useraddr = trim($useraddr);
#patterns for name, domain and top-level domains
$name = "/[-!#$%\'*+\.\/0-9=?A-Z`{|}~]+";
$host = "([-0-9A-Z]+.)+";
$tlds = "([0-9A-Z]){2,4}$/i";
#check validity of email format
if( !preg_match($name."@".$host.$_tlds,$useraddr) )
{$errmsg .="Il y a une erreur dans l'adresse email !<br>"; $valid=false;} }
#check comments field is not blank
if( !$comments )
{ $errmsg.="Vous avez oublié d'écrire votre message...<br>"; $valid = false; } }
#if invalid write the error message/s and the form
if($valid!=true)
{ echo( $errmsg.$form ); }
else #if the form is valid send the email
{ $to = "incentives@reperages-thailand.com";
$re = "Message envoyé par $username de www.incentives.reperages-thailand.com";
$msg = $comments;
$headers = "From: $useraddr \r\n";
if(mail($to,$re,$msg, $headers))
{ echo("Merci de votre message, $username."); } }
?>