Error in php/html code page not loading

Who can explain what is happening:

Using the code like this it works fine,

<?php
if  (isset($_SESSION['id'])) {
echo $_SESSION['uid'];
} else { echo "<form action='signup.php'>
                         <button class=' button button1'>SIGN UP</button>
                        </form>";
                  }

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Add tags to first echo out put and it crash error "the page isn't working".

example:

<?php
if  (isset($_SESSION['id'])) {
echo "<tag>$_SESSION['uid'];</tag>"
} else { echo "<form action='signup.php'>
                         <button class=' button button1'>SIGN UP</button>
                        </form>";
                  }

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Any help is welcome