You must update the variable with the results of the str_replace
operation like this:
<?php
include_once("php_includes/loginLinks.php");
$loginLinks = str_replace("member_profile","php_includes/member_profile",$loginLinks);
$loginLinks = str_replace("member_account","php_includes/member_account",$loginLinks);
$loginLinks = str_replace("logout","php_includes/logout",$loginLinks);
$loginLinks = str_replace("register_form","php_includes/register_form",$loginLinks);
$loginLinks = str_replace("login","php_includes/login",$loginLinks);
echo $loginLinks;
?>
str_replace
doesn't modify the original variable, but returns a new value. Here I update $loginLinks with the updated value.