gistfile1.php

<?php
    /*** turn on error reporting ***/
    error_reporting( E_ALL );

    /*** an array of test values ***/
    $values = array( $var, null, 0, false, 100, '');
    echo '<tr>';
    echo '<td>isset()</td>';
    foreach( $values as $val )
    {
        echo '<td>';
        var_dump( isset( $val ) );
        echo '</td>';
    }
    echo '</tr>';

        echo '<tr>';
    echo '<td>empty()</td>';
        foreach( $values as $val )
        {
                echo '<td>';
                var_dump( empty( $val ) );
                echo '</td>';
        }
    echo '</tr>';

        echo '<tr>';
        echo '<td>is_null()</td>';
        foreach( $values as $val )
        {
                echo '<td>';
                var_dump( is_null( $val ) );
                echo '</td>';
        }
        echo '</tr>';

        echo '<tr>';
    echo '<td>==</td>';
        foreach( $values as $val )
        {
                echo '<td>';
                var_dump( $val == false  );
                echo '</td>';
        }
    echo '</tr>';

        echo '<tr>';
    echo '<td>===</td>';
        foreach( $values as $val )
        {
                echo '<td>';
                var_dump( $val === false );
                echo '</td>';
        }
        echo '</tr>';
?>
</tbody>
</table>