<?php 
    if ($GET['page'] == 'main')
    {
        $a = 'style="text-decoration: underline;"';        
    }
    elseif($GET['page'] == 'second')
    {
        $b = 'style="text-decoration: underline;"';
    }
    elseif($GET['page'] == 'third')
    {
        $c = 'style="text-decoration: underline;"';
    }
    elseif($GET['page'] == 'four')
    {
        $d = 'style="text-decoration: underline;"';
    }
?>

<div id="menu">
<table width="1000px" border="1" height="50px" align="center" style="text-align: center;">
    <tr>
        <td>
            <a href="<?php echo href('page=main')?>" <?php echo $a;?>>Главная</a>
        </td>
            
        <td>
            <a href="<?php echo href('page=second')?>" <?php echo $b;?>>Меню2</a>
        </td>
        
        <td>
            <a href="<?php echo href('page=third')?>" <?php echo $c;?>>Меню3</a>
        </td>
        
        <td>
            <a href="<?php echo href('page=four')?>" <?php echo $d;?>>Меню4</a>
        </td>
        
        <td>
            <a href="<?php echo href('page=five')?>">Меню5</a>
        </td>
    </tr>
</table>
</div>

<?php
	$page = !empty($_GET['page']) ? $_GET['page'] : null;
	
	function pageVid($str)
	{
		global $page;
		if($page == $str)
			echo 'style="text-decoration: underline;"';
	}
?>
<div id="menu">
<table width="1000px" border="1" height="50px" align="center" style="text-align: center;">
    <tr>
        <td>
            <a href="<?php echo href('page=main')?>" <?php pageVid('main') ?>>Главная</a>
        </td>
            
        <td>
            <a href="<?php echo href('page=second')?>" <?php pageVid('second') ?>>Меню2</a>
        </td>
        
        <td>
            <a href="<?php echo href('page=third')?>" <?php pageVid('third') ?>>Меню3</a>
        </td>
        
        <td>
            <a href="<?php echo href('page=four')?>" <?php pageVid('four') ?>>Меню4</a>
        </td>
        
        <td>
            <a href="<?php echo href('page=five')?> <?php pageVid('five') ?>">Меню5</a>
        </td>
    </tr>
</table>
</div>