# cat foo
Hello, World

Will update the files atime, but it will not effect the ctime or mtime.

# chmod 777 foo

Will update the files ctime but not the files atime or mtime.

# echo “Goodbye, World!” > foo

Will update the files ctime and mtime but not the files atime.

• Connecting to your database server.
• Selecting a database.
• Executing the query.

When a query fails, many users don’t know where to begin debugging. This guide outlines my debugging procedure.

Verifying Database Connectivity

1. A database connection is generally established using mysql_connect. As the documentation states:

Returns a MySQL link identifier on success, or FALSE on failure.

Thus the first thing to check is the value mysql_connect is returning. There are several ways to test the output. Assuming you are using the following line of code to connect to your database

$link = mysql_connect('localhost', 'mysql_user', 'mysql_password');

You can do as the PHP manual suggests:

if (!$link) {
    die('Could not connect: ' . mysql_error());
}
echo 'Connected successfully';

or for something quick and dirty:

var_dump($link);

The former will output Connected successfully on success and the latter will output something similar to resource(2) of type (mysql link) If this is the case, you know mysql_connect is returning a resource and your issue probably has nothing to do with establishing a connection to the database. If the former outputs Could not connect: or the latter outputs bool(false) you have a problem connecting to your database server. In this case you should:

• Verify your parameter order is correct – server, username, password.
• Verify that the credentials are correct.
• Verify your mysql server is actually running…

If you are still having issues connecting to your database, your problem is beyond the discussion of this guide.

Database Selection

2. Once you have verified connectivity with the server, verify a database has been selected. Database selection is accomplished using mysql_select_db. This function will return true on success and false on failure. As done in the previous section we want to check which value mysql_select_db is returning. Assuming you have the following code:

$db_selected = mysql_select_db('foo', $link);

You can either use

if (!$db_selected) {
    die ('Can\'t use foo : ' . mysql_error());
}

or simply

var_dump($db_selected)

If you receive Can’t use foo or bool(false) your issue is with selecting your database.

• Verify the database you are selecting is spelled correctly.
• Verify the database exists.
• Verify the user you connected to the database server with has access to this particular database.

The Query

3. If you have successfully established a connection with the database server and have selected a database, and your query still does not work, then the problem is most likely your query itself. Assuming you use are using the following code to query your database:

$result = mysql_query('SELECT * WHERE 1=1');

You can see more information about your error using mysql_error(). Here is how:

$result = mysql_query('SELECT * WHERE 1=1');
if (!$result) {
    die('Invalid query: ' . mysql_error());
}

This will usually result in a Googleable error or you can refer to the MySQL documentation for the MySQL error codes. Unfortunately, most queries are not as simple as the one in the example above. They can usually span several lines and almost always contain variables. If this is the case, I echo my query and execute the output it in phpMyAdmin. phpMyAdmin usually gives me an error that is easily understandable, like “Unknown column name foo.”

$query = "SELECT * FROM foo WHERE id=$id";
die($query);
...

This will print your query exactly as it is executed. Many times unescaped characters in variables (which may cause SQL Injections) will cause your query to fail. Echoing your query will help catch this. Lastly, try not to use a keyword as a database name or column. For example, if you named a column select your query might look like

The error here is obvious. MySQL interpreters select as a keyword and not as a column name. To avoid this, use backticks around your table and column names.

isset()

From the PHP manual:

isset — Determine if a variable is set and is not NULL

In other words, a variable is set if it has been assigned a value other than NULL. If a variable is assigned to be an empty string – it is set. The following code and output should illustrate my point.

<?php

var_dump(isset($x));

$x = NULL;
var_dump(isset($x));

$x = "";
var_dump(isset($x));

$x = '';
var_dump(isset($x));

?>

empty()

Again from the PHP manual:

empty — Determine whether a variable is empty

In other words, a variable is empty if it is an empty string, 0, “0″, false, NULL, array(), and an unset variable are all empty.

<?php

var_dump(empty($x));

$x = NULL;
var_dump(empty($x));

$x = "";
var_dump(empty($x));

$x = '';
var_dump(empty($x));

$x = "0";
var_dump(empty($x));

$x = 0;
var_dump(empty($x));

$x = false;
var_dump(empty($x));

$x = array();
var_dump(empty($x));

?>

Now when you are validating forms to make sure a user did not leave a form field blank, it is probably best to use neither empty() or isset() (not that isset would work). Since it is possible your form might accept 0 as a valid answer. Therefore you should just check to make sure it is not an empty string.

<?php
if($_GET['foo'] == "") {
    echo "You must enter a value for foo!";
}
?>

<?php

/* create a test image */
$im = @imagecreate(100, 20) or die("Cannot Initialize new GD image stream");
$background_color = imagecolorallocate($im, 255, 255, 255);
$text_color = imagecolorallocate($im, 0, 0, 0);
imagestring($im, 1, 5, 5,  "Hello, World!", $text_color);

/***
 * Assumptions:
 *   A monochrome image where characters are black
 *   A single character is connected
 *   Characters are disjointed by white space
 */

$width = imagesx($im);
$height = imagesy($im);

/***
 * Notes:
 *   The smaller the sample size the more accurate it will be,
 *   however, it will take longer. Larger images can use a larger
 *   sample size wihtout compromizing much accuracy.
 */
$x_sample = 1;
$y_sample = 1;

$last = 0;
for($i = 0; $i < $width; $i++) {
    $col = array();

    for($j = 0; $j < $height; $j++) {
        $col[$j] = imagecolorat($im, $i, $j);
    }

    if(($current = array_sum($col)) > 0) {
        if($last == 0) {
            $l = 0;
        }

        for($k = 0; $k < $height; $k++) {
            if(($l % $x_sample) == 0) {
                if(($k % $y_sample) == 0) {
                    $sample .= $col[$k];
                }
            }
        }

        $l++;
        $last = $current;
    } else {
        $last = 0;
    }

    if(!empty($sample) && $last == 0) {
        echo $sample . "\n";
        $sample = "";
    }
}
?>

Output (each line represents a character)

Introduction:
Have you ever been on aim and received a message from one of your friends with a big long URL followed by something like “John%20Is%20Stupid.” You know what ever it is, wont be worth your time, but you click on it anyway to find someone goofy looking holding a sign and on it printed in big bold letters “John Is Stupid.” Ever wonder how they work? Chances are, the script takes advantage of the GD functions. In this tutorial you will learn many GD functions that you can use to generate images. The goal for this tutorial is for you to generate a dynamic user bar.

Here is a list of the functions used in this tutorial:

1. header()
2. imagecolorallocate()
3. imagettfbbox()
4. imagesx()
5. abs()
6. imagettftext()
7. imagepng()
8. imagedestroy()

The first thing you want to do is create the header type using the header function. Next you use the GD tools to create an image from a png file. In my case, I have userbar.png in the same directory as this script is in. The function imagecolorallocate creates a color using RGB (red-green-blue) format. The next three lines just set some basic information which isn’t that hard to understand. The last three lines to must of the work creating the image. The most important function here to pay attention to is imagettftext. The imagettftext requires 8 arguments which are the resource image, font size, the angle, x location, y location, font color, the actual font, and the string to be printed.

<?php
    header('Content-type: image/png');
    $im = imagecreatefrompng ("userbar.png");
    $color = imagecolorallocate($im, 0, 0, 0);
    $text = "test";
    $font = 'font.ttf';
    $size = 8;
    imagettftext($im, $size, 0, 15, 12, $color, $font, $text);
    imagepng($im);
    imagedestroy($im);
?>

Now that we have GD generating an image, lets make it a little more complex. We’ve added the

$text = $_GET['text'];

line to allow the user to enter a random string, and we have also added the

$size = imagettfbbox($fontsize, 0, $font, $text);
$dx = (imagesx($im)) - (abs($size[2]-$size[0])) - 20;

Which calculate where to start printing the string on the image. The function imagettfbbox generates the length of the font string in pixels. If you were to use something like strlen($text) that would just return the number of characters in the string, which isn’t very usefull in this case. Next, we use the imagesx() function to determine the width of the image, we take that value and subtract the string length in pixels and subtract an extra 20 pixels to leave some room toward the end of the image.

<?php
    header('Content-type: image/png');
    $text = $_GET['text'];
    $im = imagecreatefrompng ("userbar.png");
    $color = imagecolorallocate($im, 0, 0, 0);
    $font = 'font.ttf';
    $fontsize = 6;
    $size = imagettfbbox($fontsize, 0, $font, $text); //calculate the pixel of the string
    $dx = (imagesx($im)) - (abs($size[2]-$size[0])) - 20; //calculate the location to start the text
    imagettftext($im, $fontsize, 0, $dx, 13, $color, $font, $text);
    imagepng($im);
    imagedestroy($im);
?>

At this point you could call yourself done. However you could add some extra features such as make the font color vary with the user input. Adding a simple if statement will accomplish this.

<?php
    header('Content-type: image/png');
    $text = $_GET['text'];

    $im = imagecreatefrompng ("userbar.png");

    if($text == "ADMINISTRATOR"){ //if administrator
        $color = imagecolorallocate($im, 255, 0, 0); //red
    }
    else if($text == "MODERATOR"){ //if moderator
        $color = imagecolorallocate($im, 0, 0, 255); //blue
    }
    else if($text == "JUNKIE"){ //if junkie
        $color = imagecolorallocate($im, 0, 0, 0); //black
    }
    else { //something else...
        $color = imagecolorallocate($im, 0, 128, 0); //green
    }

    $font = 'font.ttf'; //font file
    $fontsize = 6; //font size
    $stringsize = imagettfbbox($fontsize, 0, $font, $text); //calculate the pixel of the string
    $dx = (imagesx($im)) - (abs($stringsize[2]-$stringsize[0])) - 20; //calculate the location to start the text
    imagettftext($im, $fontsize, 0, $dx, 13, $color, $font, $text);
    imagepng($im);
    imagedestroy($im);
?>

Finally you can save this script as generator.php and display it in forums by using the html tags.

I found this one just as funny, but for a different reason…

I’m just trying to figure out how a GUI interface can track an ip address – let alone something written in VB.

What does $_SERVER['REQUEST_URI'] provide us with? Essentially, everything after the domain name. So if our URL is http://www.example.com/index.php?module=Blog&view=index, our URI is index.php?module=Blog&view=index. Moreover, if our URL is http://www.example.com/index.php/Blog/index, our URI is index.php/Blog/index. Using this, we can simply manipulate the URI to process incoming requests. First, we want to explode the URI at the file name, in my case it is index.php.

$requests = explode("/index.php/", $_SERVER['REQUEST_URI']);

$request is now an array whose first element is everything before and including index.php, and whose second element is everything after index.php. By manipulating $requests[1] we can use that data as our data.

$requests = explode("/index.php/", $_SERVER['REQUEST_URI']);
$uri = explode('/', $requests[1]);

The $uri array now contains all our disguised requests, which we can use in the same way as a $_GET request (with the exception of a sexier URL).

With three lines of code we have transformed a ugly looking URL http://www.example.com/index.php?module=Blog&view=index to http://www.example.com/index.php/Blog/index (a URL the most avid Web 2.0 gurus would find acceptable).

For a signal x(t), its Laplace transform X(s) is defined by

Example: For a signal x(t) = e^-atu(t) find the Laplace transform X(s).

By definition

Since u(t) = 0 for t < 0 and u(t) = 1 for t ≥ 0,

Thus we can conclude