* Know your data. When writing the SQL statement, you should have some idea of how many rows are in each table, how selective your WHERE predicates are, and how many rows you expect in the result set. The larger the number of rows involved, the more time you should spend thinking about the best way to write the SQL statement.

* Minimize returned rows. The fewer the rows in the result set, the more efficiently the query will run.

* Avoid unnecessary columns. The wider the data in each row in the result set, the more disk space and memory that is required for intermediate operations such as sorts and to hold the result set.

* Use GROUP BY instead of DISTINCT. In most DBMSs, a GROUP BY is a more efficient way to eliminate duplicate rows compared with the DISTINCT keyword. The reason for this is that a GROUP BY invokes the sort required to find the duplicates earlier in the processing of the query, while a DISTINCT applies the sort as the very last step (applied to the final result set). The sooner the duplicate rows are eliminated, the more efficiently the remainder of the processing on that result set can be performed.

* Views may help. Views can help because they hide complex operations such as nested aggregate functions. And with DBMSs that don't have an SQL statement cache, views may process more efficiently than ordinary queries because the SQL statement that defines the view has already been parsed and optimized, which means this work does not have to be done every time the view is accessed. But above all, remember that views are also SQL queries, so they are subject to all the tuning considerations you apply to any SQL statement.

• In PHP4, objects and arrays should be passed in and out of functions by reference (with &), and everything else by value. In PHP5, objects are automatically passed by reference. For example the following gives best performance:
  function &test(&$obj_or_array)
  {
  return $obj_or_array;
  }
  $var =& test($obj);
  
  
  
• Be miserly and sparse with your server and web pages. Don't run X-Windows on the server and other unneeded processes.

In PhP, When you are working with String, if suppose want to check that the string contain certain length , you considerably use strlen() function. Of course, it is pretty quick method to calculate the length of string. However, it is somewhat slow because the function call require to refer in hash table lookup and then execute that function.

For that case, you can use isset() method to improve the execution speed of your code.

isset() is a language construct and not a function meaning that its execution doesnot refer the lookup table. This means that virtually you have no overhead on calculating the String length

Its simple, In the following Example, It check for the string having less than 15 character, if it so print 'string having less than 15 characters',

here it is....

• Don't use images when text will do. Reduce your image sizes with a software like Adobe ImageReady or MacroMedia Fireworks. Avoid dithered images as they tend to compress poorly.
  
• Spread the workload. Run your SQL server on another machine. Serve graphics and HTML from another low-end computer. If all static content is served from another server, then you can turn off KeepAlives in httpd.conf on the PHP server to speed up disconnects.
  
• Use hdparm to tune your hard disk. If you are using a default Linux install, this could speed up your hard disk by 200%. This is mostly useful for IDE hard disks, but some hdparm settings work with SCSI also.

By now nearly everyone knows you should use HEIGHT and WIDTH attributes for your image tags. Using these attributes lets the browser allocate space for your image within the page layout. That way the browser can render the rest of the page, leaving a placeholder for the image while it's loading. HEIGHT and WIDTH attributes for your TABLE and TD tags are just as important, for exactly the same reason. These attributes tell the browser how much space to allocate for each table cell. Without them, the browser waits until all the table's contents have been loaded before rendering any of its contents.

* Use the WIDTH attribute for all your table cells.
* If possible, try to use the HEIGHT attribute for table cells. But be careful with this attribute as explained below.
* Avoid nesting tables if at all possible. Nested tables can dramatically increase your rendering time. If you must nest tables, be sure to use the HEIGHT and WIDTH attributes, at least on the inner table.

Setting the WIDTH attribute for a table is most important in the first row, since the browser uses this row to set the column widths for all the other rows. Likewise setting the HEIGHT attribute is most important for the first cell within each table row, since this sets the height for the rest of that row.
Use of the HEIGHT attribute for tables and table cells is a bit risky, since you don't know the size of the browser's default font. If you plan your table layout assuming everyone is using a font size of 12 points, your page may not be rendered correctly for visitors who've changed their default font. Visitors who've set their browser font to 14 points may see text spill over from one cell to another because the text can't fit into the space you allocated.
There are two ways around this:

* Test your page with your browser's default font set to a larger value.
* Use the FONT tag and SIZE attribute to set the font size for each table cell. Use absolute values, such as "SIZE=2," not relative values, like "SIZE= -1."

1. echo() is faster than print() - print() returns a successful return code when it’s done, while echo doesn’t.
2. print(’something’) is faster than print (”something”) - since you can embed variables into double quotes, on print(”hello $some_var”) PHP parser makes an additional pass over double quotes to ensure there’s no variable to be interpreted.

• Host your website with a company known for performance. Check newsgroups and web host listing sites such as TopHosts (Web Hosting - Find Affordable Web Hosting Providers) for companies known for high performance. In a shared server environment (not dedicated hosting), find out how many other websites are on the same server as yours. Some web hosts will put up to 1 thousand sites on one server.
  
• When hosting your own website, install as much physical RAM as possible on the web server. As memory gets cheaper and cheaper by the minute it is one of the most cost effective purchases an IT department can make.

• When hosting your own website, use quality hardware ??? don t try and save money when purchasing the hardware that will run one of your most valuable assets.
  
• When hosting your own website, consider purchasing quality NIC cards which can offload and reduce CPU usage for networking.
  
• Monitor your web pages for performance to make more informed decisions. End-to-end testing of your website, e.g. testing of multiple web pages on your website, is essential to understanding weak points and places for improvement.