Are PHP script read from Top to bottom?
<p>Are PHP scripts reads from top to bottom? like HTML? because in this code</p>

<pre><code>&lt;?php require_once("./includes/connection.php")?&gt;
&lt;?php require_once("./includes/functions.inc.php"); ?&gt;
&lt;?php 

    if(isset($_GET['subj']))
    {
        $sel_subj = get_subject_by_id($_GET['subj']);
        $sel_page = NULL;

    }else if(isset($_GET['page']))
    {
        $sel_subj = NULL;
        $sel_page = get_page_by_id($_GET['page']);
    }else
    {
        $sel_subj = NULL;
        $sel_page = NULL;
    }


?&gt;
&lt;?php include("includes/header.inc.php"); ?&gt;
&lt;table id="structure"&gt;
    &lt;tr&gt;
        &lt;td id="navigation"&gt;
            &lt;ul class = "subjects"&gt;
                &lt;?php

                    $subject_set = get_all_subjects();

                    while($subject = mysql_fetch_array($subject_set))
                    {
                        echo "&lt;li";
                        if($subject['id'] == $sel_subj['id']) {echo " class =\"selected\"";}
                        echo "&gt;&lt;a href=\"content.php?subj=" . urlencode($subject["id"]) . 
                        "\"&gt;{$subject["menu_name"]}&lt;/a&gt;&lt;/li&gt;";

                        echo "&lt;ul class = 'pages'&gt;";

                        $page_set = get_pages_for_subject($subject['id']);

                        while($page = mysql_fetch_array($page_set))
                        {
                            echo "&lt;li"; 
                            if($page['id'] == $sel_page['id']){echo " class = \"selected\"";}
                            echo"&gt;&lt;a href=\"content.php?page=" . urlencode($page["id"]) .
                            "\"&gt;{$page["menu_name"]}&lt;/a&gt;&lt;/li&gt;";
                        }
                        echo "&lt;/ul&gt;";
                    }
                ?&gt;
                &lt;/ul&gt;
        &lt;/td&gt;
        &lt;td id="page"&gt;
                &lt;?php if(isset($sel_subj)){?&gt;
                    &lt;h2&gt;&lt;?php echo "{$sel_subj['menu_name']}";?&gt;&lt;/h2&gt;
                &lt;?php } ?&gt;  
                &lt;?php if(isset($sel_page)){?&gt;
                    &lt;h2&gt;&lt;?php echo "{$sel_page['menu_name']}"?&gt; &lt;/h2&gt;
                &lt;?php }?&gt;
        &lt;/td&gt;
    &lt;/tr&gt;
&lt;/table&gt;
&lt;?php require("includes/footer.inc.php"); ?&gt;
</code></pre>

<p>specifically this part</p>

<pre><code>if(isset($_GET['subj']))
        {
            $sel_subj = get_subject_by_id($_GET['subj']);
            $sel_page = NULL;

        }else if(isset($_GET['page']))
        {
            $sel_subj = NULL;
            $sel_page = get_page_by_id($_GET['page']);
        }else
        {
            $sel_subj = NULL;
            $sel_page = NULL;
        }
</code></pre>

<p>How is this if-else block being called if it's on top of the page? </p>
