I cannot resolve "redirect loop" - php

Question

This webpage has a redirect loop.
The above has been discussed and answered broadly here but, after following the recommendations found here I still cannot resolve problem with "header("Location:...)".
The code is executed when user clicks a "forgotten password" link in email. The link has random string attached to it like ".../?trs=randomstringstoredindatabase".
When code executes:
if string is valid I get the following results:
Case with code lines "header...;: & "exit;" uncommented:
URL in browser: http://localhost/pl_00_00/pl_process_forgot_password_back.php/pl_reset_password.php/index.php
Chrome displays: This webpage has a redirect loop
Case with code lines "header...;: & "exit;" commented out:
URL in browser: http://localhost/pl_00_00/pl_process_forgot_password_back.php/?trs=015df6fcf5bdcd4d9a339d5ca79d27a7 - correct
Chrome displays echo: "Redirect to: pl_reset_password.php/?memid=11" - as expected
otherwise (string not valid):
Case with code lines "header...;: & "exit;" uncommented:
URL in browser: http://localhost/pl_00_00/pl_process_forgot_password_back.php/index.php
Chrome displays: This webpage has a redirect loop
Case with code lines "header...;: & "exit;" commented out:
URL in browser: http://localhost/pl_00_00/pl_process_forgot_password_back.php/?trs=015df6fcf5bdcd4d9a339d5ca79d27a
Chrome displays echo: "Redirect to: index.php" - as expected
So, when not using "header()" function the logic seems correct. As soon as I uncomment "header()" & "exit" and comment out echos and var_dumps, the problem appears and redirections are not happening. Here is the code I use (PASSWORD_RESET_PAGE & HOME_PAGE are constants defined elsewhere and both do not have function "header ("Location...")" that could cause redirect loop in them):
<?php
# pl_process_forgot_password_back.php
require_once ("lib/required.php");
# $_GET the string from url check and if matches, active and not expired display password change form
$trs_from_email = $_GET['trs'];
# retrieve user data
$query = "SELECT * FROM tbl_member WHERE temporary_random_string = '" . $trs_from_email . "'";
$data_retrieved = sql_get_results_array($query); # retrieve data
if (db_affected_rows() == 1) { # string matched - 1 row selected
if ($data_retrieved[0]['random_string_active']){ # string active
if ($data_retrieved[0]['random_string_expiry'] > time()){ # string not expired
# all ok - display reset password form
header ( "Location: " . PASSWORD_RESET_PAGE . '/?memid=' . $data_retrieved['id'] ); # send them to page with reset-password form
exit;
echo '<br>' . '# all ok - display reset password form';
echo '<br>' . 'Redirect to: ' . PASSWORD_RESET_PAGE . '/?memid=' . $data_retrieved[0]['id'];
var_dump($_SESSION);
var_dump($data_retrieved);
die;
} else { # string expired
$_SESSION['popup_msg_id'] = 17; # this is to popup request expiry message
} # /if string not expired
} else { # string inactive
$_SESSION['popup_msg_id'] = 18; # request already processed message
} # /if string active
} else { # string not matched
$_SESSION['popup_msg_id'] = 19; # string not matched message
} # /if valid string found
header ( "Location: " . HOME_PAGE ); # send them to homepage and display popup error
exit;
echo '<br>' . '# some error';
echo '<br>' . 'Redirect to: ' . HOME_PAGE;
var_dump($_SESSION);
var_dump($data_retrieved);
die;
?>
Can anybody see why the above code is causing the "redirect loop"?

Answer 1

Keep in mind that die and exit are equivalent, so pick one.
The script will not continue to execute after die or exit is hit in the script. Because of this you are cutting off your echo statements and var_dumps, which should be before header if you want them to be seen, but a redirect can be pretty quick which means they may not be seen at all.
a redirect loop can happen if you are redirecting to the same page that you are currently on or if you keep redirecting the user on subsequent pages. If you are sure this is not the case then you need to remove all the cookies and browser cache associated with your site which should fix the problem.
The following sample and place your code in the correct order.
<?php
# pl_process_forgot_password_back.php
require_once ("lib/required.php");
# $_GET the string from url check and if matches, active and not expired display password change form
$trs_from_email = $_GET['trs'];
# retrieve user data
$query = "SELECT * FROM tbl_member WHERE temporary_random_string = '" . $trs_from_email . "'";
$data_retrieved = sql_get_results_array($query); # retrieve data
if (db_affected_rows() == 1) { # string matched - 1 row selected
if ($data_retrieved[0]['random_string_active']){ # string active
if ($data_retrieved[0]['random_string_expiry'] > time()){ # string not expired
# all ok - display reset password form
echo '<br>' . '# all ok - display reset password form';
echo '<br>' . 'Redirect to: ' . PASSWORD_RESET_PAGE . '/?memid=' . $data_retrieved[0]['id'];
var_dump($_SESSION);
var_dump($data_retrieved);
header ( "Location: " . PASSWORD_RESET_PAGE . '/?memid=' . $data_retrieved['id'] ); # send them to page with reset-password form
exit;
} else { # string expired
$_SESSION['popup_msg_id'] = 17; # this is to popup request expiry message
} # /if string not expired
} else { # string inactive
$_SESSION['popup_msg_id'] = 18; # request already processed message
} # /if string active
} else { # string not matched
$_SESSION['popup_msg_id'] = 19; # string not matched message
} # /if valid string found
echo '<br>' . '# some error';
echo '<br>' . 'Redirect to: ' . HOME_PAGE;
var_dump($_SESSION);
var_dump($data_retrieved);
header ( "Location: " . HOME_PAGE ); # send them to homepage and display popup error
exit;
?>