need help with php

[awesomeamateurs]

working on my cms and i keep getting

Fatal error: Call to undefined function get_subject_by_id() in C:\wamp\www\mega_site\content.php on line 14

01 <?php require_once("includes/connection.php"); ?>
02 <?php require_once("includes/header.php"); ?>
03 <?php
04    if (isset($_GET['subj'])) {
05     $sel_subj = $_GET['subj'];
06     $sel_page = "";
07    } elseif (isset($_GET['page'])) {
08      $sel_subj = "";
09        $sel_page = $_GET['page'];
10    } else {
11        $sel_page = "";
12        $sel_subj = "";
13    }    
14    $sel_subject = get_subject_by_id($sel_subj);
15 ?>
16<?php include("includes/functions.php"); ?>
17<table id="structure">
18    <tr>
19        <td id="navigation">
20        <ul class="subjects">
21    
        <?php
    $subject_set = get_all_subjects();
    while ($subject = mysql_fetch_array($subject_set)) {
        echo "<li"; 
        if ($subject['id'] == $sel_subj) { echo " class=\"selected\""; }
            echo "><a href=\"content.php?subj=" . urlencode($subject["id"]) . 
            "\">{$subject["menu_name"]}</a></li>";
        $page_set = get_pages_for_subject($subject["id"]);
        echo "<ul class=\"pages\">";
        while ($page = mysql_fetch_array($page_set)) {
          echo "<li";    
          if ($page['id'] == $sel_page) { echo " class=\"selected\""; }
          echo "><a href=\"content.php?page=" . urlencode($page["id"]) .
            "\">{$page["menu_name"]}</a></li>";
        }
        echo "</ul>";
    }
    ?>
        </ul>
        </td>
        <td id="page">
            <h2><?php echo $sel_subject['menu_name']; ?></h2>
                <br />
                <?php echo $sel_page; ?><br />
                    
        </td>
    </tr>
</table>
<?php require("includes/footer.php"); ?>

[Allli]

is get_subject_by_id() defined in includes/functions.php?
if so, move the include call earlier (before you call the get_subject.. function)

This should probably work:

 <?php 
  require_once("includes/connection.php"); 
  require_once("includes/header.php"); 
  include("includes/functions.php");

    if (isset($_GET['subj'])) {
     $sel_subj = $_GET['subj'];
     $sel_page = "";
    } elseif (isset($_GET['page'])) {
      $sel_subj = "";
        $sel_page = $_GET['page'];
    } else {
        $sel_page = "";
        $sel_subj = "";
    }    
    $sel_subject = get_subject_by_id($sel_subj);

//...
?>

[justFred]

yes that is most definitely your problem. you should include functions.php before you can a function inside it.