PHP echo $_SERVER['PHP_SELF'] with added variable?

Just want to do $_SERVER['PHP_SELF'] as a link with ?logout=1 appended to it.

<a href="<?php echo ''$_SERVER['PHP_SELF']'.logout=1' ?>" id="add"><input type="button" value="LOGOUT" /></a>
 

gives

Parse error: syntax error, unexpected T_VARIABLE, expecting ',' or ';' in F:\export\srv\www\vhosts\main\htdocs\php\assign3\m_a2_functions.php on line 90
 

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