Single-quoted strings
print 'I have gone to the store.';
print 'I\'ve gone to the store.';
print 'Would you pay \$1.75 for 8 ounces of tap water?';
print 'In double-quoted strings, newline is represented by \n';
Output
I have gone to the store.
I've gone to the store.
Would you pay $1.75 for 8 ounces of tap water?
In double-quoted strings, newline is represented by \n
Because PHP doesn’t check for variable interpolation or almost any escape sequences in single-quoted strings, defining strings this way is straightforward and fast.
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Posted by admin | Posted in PHP, Theory Subjects | Posted on 18-11-2009
0
1: <!DOCTYPE html PUBLIC
2: “-//W3C//DTD XHTML 1.0 Strict//EN”
3: “http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd”>
4: <html>
5: <head>
6: <title>Calling a Function Dynamically</title>
7: </head>
8: <body>
9: <div>
10: <?php
11: function sayHello() {
12: print “hello<br />”;
13: }
14: $function_holder = “sayHello”;
15: $function_holder();
16: ?>
17: </div>
18: </body>
19: </html>
A string identical to the name of the sayHello() function is assigned to the $function_holder variable on line 14. After this is done, we can use this variable in conjunction with parentheses to call the sayHello() function. We do this on line 15.
Why would we want to do this? In the example, we simply made more work for ourselves by assigning the string "sayHello" to $function_holder. Dynamic function calls are useful when you want to alter program flow according to changing circumstances. We might want our script to behave differently according to a parameter set in a URL’s query string, for example. We could extract the value of this parameter and use it to call one of a number of functions.
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Posted by admin | Posted in Theory Subjects, XML | Posted on 17-11-2009
0
If you look at the first and last lines of the code for the last example, you will see pairs of angle brackets containing the letters <html>. The two brackets and all of the characters between them are known as a tag, and there are lots of tags in the example. All of the tags in this example come in pairs; there are
opening tags and closing tags. The closing tag is always slightly different than the opening tag in that it has a forward slash character before the characters </html>.
html
The special meaning these tags give is a description of the structure of the document. The opening tag says “This is the beginning of a heading” and the closing tag says “This is the end of a heading.”Without the markup, the words in the middle would just be another bit of text; it would not be clear that they formed the heading.
Now look at the paragraph of text about the company; it is held between an opening <p> tag and a closing </p> tag. And, you guessed it, the p stands for paragraph.
As you can see, the markup in this example actually describes what you will find between the tags, and the added meaning the tags give is describing the structure of the document. For example, between the opening <p> and closing </p> tags are paragraphs and between the <h1> and </h1> tags is a heading. Indeed, the whole HTML document is contained between opening <html> and closing </html> tags.
If you were wondering why there is a number 1 after the h , it is because in HTML and XHTML there are six levels of headings. A level 1 heading is sometimes used as the main heading for a document (such as a chapter title), which can then contain subheadings, with level 6 being the smallest. This allows you to structure your document appropriately with subheadings under the main heading.
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HTML, or Hypertext Markup Language, is the most widely used language onWeb. As its name suggests, HTML is a markup language, which may sound complicated, although really you come across markup every day. Markup is just something you add to a document to give it special meaning; for example, when you use a highlighter pen you are marking up a document. When you are marking up a document for the Web, the special meaning you are adding indicates the structure of the document, and the markup indicates which part of the document is a heading, which parts are paragraphs, what belongs in a table, and so on. This markup in turn allows a Web browser to display your document appropriately.
When creating a document in a word processor, you can distinguish headings using a heading style (usually with a larger font) to indicate which part of the text is a heading. You can use the Enter (or Return) key to start a new paragraph. You can insert tables into your document, create bulleted lists,
and so on. When marking documents up for the Web you are performing a very similar process. HTML and XHTML are the languages you use to tell aWeb browser where the heading is for aWeb page, what is a paragraph, what is part of a table and so on, so it can structure your document and render it properly. But what is the difference between HTML and XHTML?Well, first you should know that there are several versions of both HTML and XHTML, but don’t let that bother you—it all sounds a lot more complicated than it really is. Whereas there are several versions of HTML, each version just adds functionality on top of its predecessor (like a new version of some software might add some features or a new version of a dictionary might add a few extra words), or offers better ways of doing things that were already in earlier versions. So, you do not need to learn each version of HTML and XHTML, nor do you need to focus on one variation. This book teaches you all you need to know to writeWeb pages using HTML and XHTML. Indeed, as I mentioned in the Introduction, XHTML is just like the latest version of HTML, as you will see shortly (although to be accurate, while it is almost identical to the last version of HTML, it is technically HTML’s successor).
Sample Code :
<html>
<head>
<title>Vdiscussion: About Us</title>
</head>
<body>
<h1>About Vdiscussion.com</h1>
<p>Vdiscussion is free educational site
</p>
</body>
</html>
Save this code as filename.html.
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Posted by Dora_david | Posted in Client/Server, Theory Subjects | Posted on 16-11-2009
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A three-tier (or n-tier) application is when the application components are spread out over three or more computers and there is a high degree of separation between the user interface, business logic and data access, and the data components. If there is no degree of separation, you are bound to have a failed application when it is time to perform maintenance. This separation is referred to as a loosely coupled design.
Three-Tier-Architecture
that the center node is called the Application Server and that it hosts, in the logical design, only the business logic components. In any application, the application is the part of the system that controls the application’s logic. The database is just a place to store the data and the user interface is just the means to get that data there, but the business logic contains all of the functionality for dealing with data. As you start writing this application, you will see that the business logic is the most important component of any application.
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Usually it is when there are only going to be a small number of users who will ever use the application. When I say small number, I mean about 100 or fewer users. Another time to use two-tier architecture is when other applications will not need to access the functionality provided by the two-tier application. Take for instance an application that performs some function that is only needed in this one instance—you probably will not need to worry about incorporating this functionality into other applications. Because the functionality does not need to be reused, there is no point in creating a reusable component.
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Posted by ma.vinothkumar | Posted in Uncategorized | Posted on 16-11-2009
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When multiple users try to update a single record at the same time, you will have concurrency issues. In a single-tier application, this is never an issue because there is only one user accessing data. In a two-tier system, you as the developer have the option of implementing either pessimistic or optimistic locking—however, this depends on the Relational Database Management System (RDBMS) because not all databases support optimistic locking. In most two-tier applications, you are always connected to the database when you are reading and writing data. If you set up pessimistic concurrency, when one user is trying to update a record, no other user can update the record at the same time (they will receive an error message explaining that the record is locked). If you implement optimistic concurrency, you will have to write code to handle the occurrence of one user updating a record that is not current. In a three-tier application, the developer must always handle database updates because no connection is maintained with the database. In most cases, this applies to a well-written two-tier application as well.
Microsoft’s new database access technology, ADO.NET, can help to make many of these issues easier to solve—but even ADO.NET will only throw an exception saying that someone else has updated the record. It is still up to the developer to handle this situation, and it is rarely solved the same way on any two applications. How you handle this situation depends mostly on what the users want.
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The two-tier architecture is the most predominate architecture in corporate America. Although the Internet is slowly changing this model, do not be fooled into thinking an Internet application is not also a two-tier architecture—it depends on how it is written. Simply put, a two-tier architecture is one where the application runs on the user’s machine and the data is stored in a central location on a network
Two-Tier-Architecture
By its very nature, a two-tier application is not scalable beyond a certain point. There are several reasons for this. One reason is the number of connections a database can maintain concurrently. Imagine that one million users try to access the database at the same time…need I say more? There is no way to effectively manage the connections to the database when the connections are being created on the user’s machine (as opposed to being able to pool database connections). Another reason why a two-tier application is not scalable beyond a certain point is application functionality in relationship to the business process that the application supports. Take a situation where a business process changes and the program has to be altered. The company may have to roll the upgraded application out to 30,000 users, which is usually too cost prohibitive to do. Scalability does not have to just reflect whether an application can support a growing number of users but also how expensive it is to support them.
Then there is the concurrency issue; that is, what happens when two or more users try to access the same record in the same database at the same time to make changes to it? Usually one or more users are blocked from making changes, which can cause the application to temporarily hang. In a two-tier application, this can be both a positive and a negative aspect of the application. The positive aspect is that one user cannot alter a record that another user is modifying. The negative aspect is that it can cause the second user’s query to wait if there is a lock on the record they want to read. If the application is programmed correctly, the lock should not last for more than a few milliseconds, but on some database platforms, if the user who placed the lock is prematurely disconnected from the database, the result is a lock that cannot be removed except by the database administrator or by the database after a certain period of time. This has the potential to cause numerous problems. This particular issue is never a problem with a three-tier application, but other, more complicated issues appear with regard to this aspect of the database.
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Posted by albert | Posted in PHP, Theory Subjects | Posted on 16-11-2009
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Multitasking is the ability of an operating system to run multiple programs concurrently. Basically, the operating system uses a hardware clock to allocate “time slices” for each currently running process. If the time slices are small enough—and the machine is not overloaded with too many programs trying to do something—it appears to a user as if all the programs are running simultaneously.
Multitasking is nothing new. On large mainframe computers, multitasking is a given. These mainframes often have hundreds of terminals attached to them, and each terminal user should get the impression that he or she has exclusive access to the whole machine. In addition, mainframe operating systems often allow users to “submit jobs to the background,” where they are then carried out by the machine while the user can work on something else.
Multitasking on personal computers has taken much longer to become a reality. But we now often seem to take PC multitasking for granted. As I’ll discuss shortly, to some extent the earlier 16-bit versions of Microsoft Windows supported multitasking but in a somewhat limited capability. The 32-bit versions of Windows all support both true multitasking and—as an extra bonus—multithreading.
Multithreading is the ability for a program to multitask within itself. The program can split itself into separate “threads” of execution that also seem to run concurrently. This concept might at first seem barely useful, but it turns out that programs can use multithreading to perform lengthy jobs in the background without requiring the user to take an extended break away from their machines. Of course, sometimes this may not be desired: an excuse to take a journey to the watercooler or refrigerator is often welcome! But the user should always be able to do something on the machine, even when it’s busy doing something else.
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Posted by albert | Posted in PHP, Request | Posted on 16-11-2009
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A simple rule is, “Always use `0′ or `NULL’ for null pointers, and always cast them when they are used as arguments in function calls.”
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