This section gathers many common errors that you may face while writing PHP scripts.
PHP is a glue that brings together hundreds of external libraries, so sometimes this gets messy. However, a simple rule of thumb is as follows:
Array function parameters are ordered as "needle, haystack" whereas String functions are the opposite, so "haystack, needle".
PHP offers many predefined variables, like the superglobal $_POST. You may loop through $_POST as it's an associate array of all POSTed values. For example, let's simply loop through it with foreach, check for empty() values, and print them out.
<?php
$empty = $post = array();
foreach ($_POST as $varname => $varvalue) {
if (empty($varvalue)) {
$empty[$varname] = $varvalue;
} else {
$post[$varname] = $varvalue;
}
}
print "<pre>";
if (empty($empty)) {
print "None of the POSTed values are empty, posted:\n";
var_dump($post);
} else {
print "We have " . count($empty) . " empty values\n";
print "Posted:\n"; var_dump($post);
print "Empty:\n"; var_dump($empty);
exit;
}
?>
Note: Superglobals: availability note
Superglobal arrays such as $_GET, $_POST, and $_SERVER, etc. are available as of PHP 4.1.0. For more information, read the manual section on superglobals
Assuming this is for a database, use the escaping mechanism that comes with the database. For example, use mysql_real_escape_string() with MySQL and pg_escape_string() with PostgreSQL. There is also the generic addslashes() and stripslashes() functions, that are more common with older PHP code.
Note: directive note: magic_quotes_gpc
The magic_quotes_gpc directive defaults to on. It essentially runs addslashes() on all GET, POST, and COOKIE data. stripslashes() may be used to remove them.
Most likely the backslashes magically exist because the PHP directive magic_quotes_gpc is on. This is an old feature of PHP, and should be disabled and not relied upon. Also, the PHP function stripslashes() may be used to strip the backslashes from the string.
Note: directive note: magic_quotes_gpc
The magic_quotes_gpc directive defaults to on. It essentially runs addslashes() on all GET, POST, and COOKIE data. stripslashes() may be used to remove them.
First, an explanation about what this ini setting does. Let's say the following URL is used: http://example.com/foo.php?animal=cat and in foo.php we might have the following PHP code:
<?php
// Using $_GET here is preferred
echo $_GET['animal'];
// For $animal to exist, register_globals must be on
// DO NOT DO THIS
echo $animal;
// This applies to all variables, so $_SERVER too
echo $_SERVER['PHP_SELF'];
// Again, for $PHP_SELF to exist, register_globals must be on
// DO NOT DO THIS
echo $PHP_SELF;
?>
The code above demonstrates how register_globals creates a lot of variables. For years this type of coding has been frowned upon, and for years it's been disabled by default. Note that PHP will eventually remove this deprecated feature. So although most web hosts disable register_globals, there are still outdated articles, tutorials, and books that require it to be on. Plan accordingly.
See also the following resources for additional information:
Note:
In the example above, we used an URL that contained a QUERY_STRING. Passing information like this is done through a GET HTTP Request, so this is why the superglobal $_GET was used.
<?php
function myfunc($argument)
{
echo $argument + 10;
}
$variable = 10;
echo "myfunc($variable) = " . myfunc($variable);
?>
To be able to use the results of your function in an expression (such as concatenating it with other strings in the example above), you need to return the value, not echo it.
<pre>
<?php echo "This should be the first line."; ?>
<?php echo "This should show up after the new line above."; ?>
</pre>
In PHP, the ending for a block of code is either "?>" or "?>\n" (where \n means a newline). So in the example above, the echoed sentences will be on one line, because PHP omits the newlines after the block ending. This means that you need to insert an extra newline after each block of PHP code to make it print out one newline.
Why does PHP do this? Because when formatting normal HTML, this usually makes your life easier because you don't want that newline, but you'd have to create extremely long lines or otherwise make the raw page source unreadable to achieve that effect.
The functions header(), setcookie(), and the session functions need to add headers to the output stream but headers can only be sent before all other content. There can be no output before using these functions, output such as HTML. The function headers_sent() will check if your script has already sent headers and see also the Output Control functions.
The getallheaders() function will do this if you are running PHP as an Apache module. So, the following bit of code will show you all the request headers:
<?php
$headers = getallheaders();
foreach ($headers as $name => $content) {
echo "headers[$name] = $content<br />\n";
}
?>
See also apache_lookup_uri(), apache_response_headers(), and fsockopen()
The security model of IIS is at fault here. This is a problem common to all CGI programs running under IIS. A workaround is to create a plain HTML file (not parsed by PHP) as the entry page into an authenticated directory. Then use a META tag to redirect to the PHP page, or have a link to the PHP page. PHP will then recognize the authentication correctly. With the ISAPI module, this is not a problem. This should not effect other NT web servers. For more information, see: » http://support.microsoft.com/kb/q160422/ and the manual section on HTTP Authentication .
You have to change the Go to Internet Information Services. Locate your PHP file and go to its properties. Go to the File Security tab, Edit -< Anonymous access and authentication control.
You can fix the problem either by unticking Anonymous Access and leaving Integrated Window Authentication ticked, or, by ticking Anonymous Access and editing the user as he may not have the access right.
In order to embed <?xml straight into your PHP code, you'll have to turn off short tags by having the PHP directive short_open_tags set to 0. You cannot set this directive with ini_set(). Regardless of short_open_tags being on or off, you can do something like: <?php echo '<?xml'; ?>. The default for this directive is On.
Read the manual page on predefined variables as it includes a partial list of predefined variables available to your script. A complete list of available variables (and much more information) can be seen by calling the phpinfo() function. Be sure to read the manual section on variables from outside of PHP as it describes common scenarios for external variables, like from a HTML form, a Cookie, and the URL.
Note: register_globals: important note
As of PHP 4.2.0, the default value for the PHP directive register_globals is off. The PHP community discourages developers from relying on this directive, and encourages the use of other means, such as the superglobals.
There are a few alternatives written in PHP such as » FPDF and » TCPDF.
There is also the Haru extension that uses the free libHaru external library.
It's important to realize that the PHP directive register_globals also affects server and environment variables. When register_globals = off (the default is off since PHP 4.2.0), $DOCUMENT_ROOT will not exist. Instead, use $_SERVER['DOCUMENT_ROOT'] . If register_globals = on then the variables $DOCUMENT_ROOT and $GLOBALS['DOCUMENT_ROOT'] will also exist.
If you're sure register_globals = on and wonder why $DOCUMENT_ROOT isn't available inside functions, it's because these are like any other variables and would require global $DOCUMENT_ROOT inside the function. See also the manual page on variable scope. It's preferred to code with register_globals = off.
Note: Superglobals: availability note
Superglobal arrays such as $_GET, $_POST, and $_SERVER, etc. are available as of PHP 4.1.0. For more information, read the manual section on superglobals
The available options are K (for Kilobytes), M (for Megabytes) and G (for Gigabytes; available since PHP 5.1.0), these are case insensitive. Anything else assumes bytes. 1M equals one Megabyte or 1048576 bytes. 1K equals one Kilobyte or 1024 bytes. You may not use these shorthand notations outside of php.ini, instead use an integer value of bytes. See the ini_get() documentation for an example on how to convert these values.
Note: kilobyte versus kibibyte
The PHP notation describes one kilobyte as equalling 1024 bytes, whereas the IEC standard considers this to be a kibibyte instead. Summary: k and K = 1024 bytes.
Prior to PHP 5.3.4, there was a bug in the network resolving code inside PHP that caused localhost in all stream related situations to fail if IPv6 was enabled. To work around this issue you can either use "127.0.0.1" or disable IPv6 resolving in the hosts file.