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To make the mode "Dynamic" at the login screen you need to edit the ./imp/login.php file.  Search for the code that looks like this:

<pre><code class="language-php"> 
/* If DIMP/MIMP are available, show selection of alternate views. */

\$views = array();
if (!empty(\$conf['user']['select\_view'])) \{
    \$apps = \$registry->listApps(null, true);
    if (isset(\$apps['dimp']) || isset(\$apps['mimp'])) \{
        \$views[] = array('sel' => false, 'val' => 'imp', 'name' => \_("Traditional"));
        if (isset(\$apps['dimp'])) \{
            \$views[] = array('sel' => true, 'val' => 'dimp', 'name' => \_("Dynamic"));
        \}
        if (isset(\$apps['mimp'])) \{
            \$views[] = array('sel' => false, 'val' => 'mimp', 'name' => \_("Minimalist"));
        \}
    \}
\}
</code></pre>
Set the value for "Dynamic" to true as in the example above

Alternately, in order to not override mobile phone detection, change:

<pre><code class="language-php">
        : (\$browser->isMobile() \&\& isset(\$apps['mimp']) ? 'mimp' : 'imp');
</code></pre>
To:

<pre><code class="language-php">
        : (\$browser->isMobile() \&\& isset(\$apps['mimp']) ? 'mimp' : 'dimp');
</code></pre>
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