<form action="" method="post" class="form-inline">
    <div class="input-group">
      <input class="form-control" type="date"name="baslangic_tarihi" >
    </div>
  <div class="form-group">
    <label class="text" for="exampleInputPassword2">ve</label>
    <input type="date" class="form-control" name="bitis_tarihi">
  </div>
  <button type="button" class="btn btn-default">
  <span class="glyphicon glyphicon-search"></span> Arama yap</button>
</form>
</div>
<?php
$baslangic_tarihi = "sip_tarihi";
$bitis_tarihi = "sip_tarihi";
$sqlim = "SELECT * FROM siparis WHERE sip_tarihi BETWEEN  $baslangic_tarihi and $bitis_tarihi ";    
$yazdir = mysql_query($sqlim);
 
 
?>