* Introduce your PHP-code with <?php because a simple <? may cause interference with XML-code.

* Prefer '.....' to "....." because the server side PHP engine parses everything it finds between the " " while it doesn't control what is written between the two single quotes.
But be careful if you use variables. You have to write

PHP Code:

<?php
echo 'This is my var:', $var, '!!';
?>

instead of

PHP Code:

<?php
echo "This is my var: $var !!";
?>

By the way: <?php echo 'This is my var:',$var,'!!'; ?> should be some milliseconds faster than <?php echo 'This is my var:'.$var.'!!'; ?> (concatenated by a dot instead of the comma) and faster than <?php print 'This is my var:'.$var.'!!'; ?>

* Use SWITCH instead of IF when you have many options because it makes a lookup table and goes straight to the right case.

* Use IF instead of SWITCH if you have only few options because it evaluates every option until it finds a match.
Sometimes a trinary expression may simplifiy an IF/ELSE-expression because it reduces the written code:

PHP Code:

<?php
echo ($var==1)?'var is 1':'var is not 1';
?>

instead of

PHP Code:

<?php
if ($var==1){
    echo 'var is 1';
}
else{
    echo 'var is not 1';
}
?>

* Define the max-variables you use in your FOR-loops before starting the loop.
Prefer:

PHP Code:

<?php
$max=filesize('myfile.dat'); //just to make an example
for ($i=0; $i<$max; $i++){
//your code
}
?>

to

PHP Code:

<?php
for ($i=0; $i<filesize("myfile.dat"); $i++)
?>

So $max will be defined only once (and not as often as the loop is executed).

Use HTML instead of PHP whenever possible. So you gain speed because the server side PHP engine parses only what it finds between <?php and ?>:

HTML Code:

<h3> Today </h3> <p>Today I met <?php echo $person; ?>. That was nice.</p>

instead of

PHP Code:

<?php
echo  "<h3>Today</h3><p>Today I met $person. That was nice.";
?>

* Select only the columns you need for your query. If you really don't want to select ALL the columns of your table don't digit SELECT * FROM tablename but write all the column names instead of "*" even if this means more writing work.

* Sort your data only if it's really necessary. It wastes a lot of time.

* Keep your filter conditions as easy as possible.

* Avoid mathematic MySQL-operations if possible. Leave them to PHP.

* Use connect only if you are sure that your pages will be connected only once and your user won't reconnect.

* Use pconnect for a persistant connection if you have several queries on your page.

* Use my_fetch_row() instead of my_fetch_array(). Sometimes this may be faster (I read somewhere).
my_fetch_rows() gives back an enumerated array:

PHP Code:

<?php
while($row = mysql_fetch_rows($res)) echo $row[0], $row[1];
?>

my_fetch_array() is its extended version (the result array works with innumeric indices like above and with column names):

PHP Code:

<?php
while($row = mysql_fetch_array($res))
    echo $row['columnname0'], $row['columnname1'];
?>

No idea about mysql_fetch_object() which works only with column names:

PHP Code:

<?php
while($row = mysql_fetch_object($res))
    echo $row->columnname0, $row->columnname1;
?>

It's a Technique to reduce load on backend servers by the way of serving up all static data requests(static HTML, Javascript and Image files).

Catching is particularly optimized for fast HTTP request handling. Thus when multiple clients request a certain object, such as an image file, the client will receive the response from that cache instead of your web servers.

The ability of Cache-Content is determined by the file's cache-control.

This is one of the technique to improve the website performance.
Whenever a browser encounters javascript, it halts the stack of Yet to be performed HTTP Requests until the script has finished parsing and executing.

From the users perspective view, the page starts rendering as soon as possible, even when all images haven't loaded yet. In order to acheive this, we need to make sure that CSS get loaded as EARLY as possible and Javascript as LATE as Possible. so move all <script> tags to bottom of the page instead of head section where they usually have.

Not all JS can be moved to the bottom of page. The script needs to be written as proper progressive enhancement for this technique to work.

This is also one of the reason for to reduce the performance of the website and of course you might think its an unusual thing.
But it produce to creating the Unnecessary HTTP Request and wasting time in Javascript Execution.
Here Unnecessary HTTP Request doenot occur in firefox provided the script is cacheable but it happens in IE.

In IE, if an External script is included twice means, and that the script is not cacheable, it generates two HTTP Request during page loading. Even the script is cacheable means extra HTTP Request occur when the user reloads the page.
In addition to generating waste HTTP Request, time is wasted in evaluating the Script multiple times.

There are two possible situation for this to happen.

1. Team size
2. Number of Scripts

So to avoid this kind of situation, we need to maintain the script redundancy check system

This will speed up your PHP code by 5-15% if you frequently print or echo in your code. When output buffering is enabled it pipes the output into a dynamically growing buffer and sends the buffer content with the output in one shot at the end of the script (or when you decide). Output buffering reduces networking overhead substantially at the cost of more memory and an increase in latency.

These are the main functions to control output buffering:
* ob_start() enables output buffering. (Output buffering supports multiple levels, that means you can call ob_start() several times.)
* ob_end_flush() sends the output buffer and disables output buffering.
* ob_end_clean() cleans the output buffer without sending it and disables output buffering.
* ob_get_contents() returns the current output buffer as a string that you can echo to send the accumulated output to the browser (after turning buffering off).
* int ob_get_length() returns the length of the output buffer.

An example:

PHP Code:

<?php
ob_start(); //start buffering
ob_implicit_flush(0); //turn off implicit flushing
//now your output stuff:
echo 'This is the 1st line with compressed output.';
$str='<br /> This is the 2nd line.';
echo $str;
$var1=10;
$var2=7;
$var3=$var1-$var2;
echo '<br /> This is the ',$var3, 'rd line.';
$contents = ob_get_contents(); //if you don't add "ob_end_clean()" all that you
wanted to echo out will be echoed out NOW
ob_end_clean(); //if you add this line nothing will be echoed out, only if you add after this line "echo $contents" the content will be echoed out.
?>

If your version of PHP is compiled with zlib support and is 4.0.4 or higher, you can also try this:

PHP Code:

<?php
ob_start('ob_gzhandler');
// rest of your script
?>

1. General rule of thumb for hardware upgrades: For PHP scripts, the main bottleneck is the CPU. For static HTML/images, the bottleneck is RAM and the network. According to Compaq benchmarks in 1999 (the original article is lost due to bitrot), a slow 400 Mhz Pentium can saturate a T3 line (that's 45 Mbps) with static HTML pages.
   
2. A PHP script will be served at least 2-10 times slower than a static HTML page by Apache. Try to use more static HTML pages and fewer scripts.

Many browsers support transparent compression using gzip. The server detects that the browser supports gzip encoding, compresses and sends the data (using the Content-Encoding: gzip header line) in spite of sending data in plain text form. When the compressed data arrives at the browser, it will be decompressed. This shortens the load time for the page.
Here is the code with the function that compresses the output:

PHP Code:

<?php
function compress_output($contents){
//tell the browser that gzip data will be sent:
header("Content-Encoding: gzip");
echo "\x1f\x8b\x08\x00\x00\x00\x00\x00";
$Size = strlen($contents); 
/* figure out the size and CRC (Cyclic Redundancy
Check) of the original for later, if you want to know more about this read
Luis Argerich's article cited at the bottom of this chapter. That's where I found this code */
$Crc = crc32($contents);
$contents = gzcompress($contents, 9); // compress the data
echo $contents;
gzip_PrintFourChars($Crc);
gzip_PrintFourChars($Size);

function gzip_PrintFourChars($Val) {
for ($i = 0; $i < 4; $i ++) {
echo chr($Val % 256);
$Val = floor($Val / 256);
}
}
}
//check if the browser supports gzip encoding:
if(ereg('gzip, deflate',$HTTP_ACCEPT_ENCODING)){
compress_output();
}
else{
echo $contents;
}
?>