<FORM method="post" action="cible2.php">
     <select name="section_principale" label="Section" style="margin-top:10px" class="formulaire" id="section_principale"  tabindex="3">
		<?php

                $requete2="SELECT section FROM gest_section ORDER BY section";
                $result2=mysql_query($requete2);
                while ($ligne=mysql_fetch_array($result2))
                {
                   echo '<OPTION VALUE="'.$ligne["section"].'">'.$ligne["section"].'</OPTION>';
                }
          ?>
            
           
      </SELECT>
      <input type="submit" value="Valider" />

      </FORM>

while(...){
  echo '<input...';
  if( $valeurDuWhile == $valeurAComparer )
    echo ' selected="selected"';
  echo ' />';
}