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Well I have have already read the cannot modify header

But still I'm getting the error

Warning: Cannot modify header information - headers already sent by (output started at C:\xampp\htdocs\xampp\web\librarian_menu.php:582)

This is my code , I don't know what i am doing wrong

<body>
<?php
    @session_start();
    require 'connect.php';
    if(isset($_SESSION['lib']))
    {
        echo '<strong style="position:relative;top:25px; left:940px;">'.$_SESSION['lib'].'</strong>';
    }
    else
    {
        echo "<script>alert('You need to login first')</script>";
        header('Location: quote.php');
    }

    if(isset($_POST['log_out']))
    {
        unset($_SESSION['lib']);
        header('Location: quote.php');
    }
?>
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.
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Community
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mindalkar
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2 Answers2

1

If what you posted above really is your code, then it is pretty obvious where the problem comes from. Just look at the first line of code right before your opening php tag: 

<body>
<?php
@session_start();
require 'connect.php';

See the <body> tag? You output it before anything else. A similar issue exists within the else branch of the first conditional: first you echo some string, then you call the header() function. You simply must not output / echo anything prior to calling that header() function.

Note that such code might work, when the http server caches the preliminary output. But you have no guarantee for that. So you cannot rely on it. Apparently in your case that output is not cached, but sent before you cann the header() function.

The reason for this behaviour of php, for this issue is a simple one: http headers are preceding an http reply. Per definition there cannot be any content contained in a reply before the headers. So once php start to send any content it must close the headers and sent them first. If you try to add any additional headers afterwards php has no choice but to raise an error: sending simply is impossible within that reply.

You have to restructure your code to sent the headers prior to anything else. If that really is not possible, then you might want to take a look at "output buffering". It allows you to hold back any output and release it to the client only later, after having done whatever is required, for example defining additional http headers.

Any maybe a side note, just to make things crystal clear: with the message "Warning: Cannot modify header information..." php refers to the http headers, not to any html header tag you might use.

arkascha
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0

Before header function (such as session_start, header) you cannot output anything to the browser - remove echos and html tags like <body> from the code before session_start() or header().

Read the first few lines of this : http://php.net/manual/en/function.header.php

n-dru
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