The main factor of the PHP language is practicality(usefulness). PHP should provide the programmer with the tools to quickly and efficiently solve the tasks at hand. 

The Important Characteristics are:

There’s another “characteristic or we can say main advantage” that makes PHP especially attractive: it’s free! Moreover, with open-source codes (Open Source means the original source code for free for everyone).

Traditional

The PHP language will feel familiar to programmers working in different fields. Many language constructs are taken (borrowed) from C, Perl.

PHP code is very similar to that found in typical C or Pascal programs. This significantly reduces the initial effort in learning PHP. PHP is a language that combines the virtues of Perl and C and is specifically aimed at working on the Internet, a language with a universal (albeit with some reservations) and clear syntax.
And although PHP is a fairly youthful language, it has gained similar popularity among web programmers that at the moment it’s almost the most popular language for creating web applications (scripts).

Simplicity

A PHP script can be 10,000 lines long or one line long, depending on the specifics of your task. You don’t have to load libraries, specify special compilation options or anything like that. The PHP engine simply starts executing code after the first escape sequence (<?) and continues execution until it encounters the paired escape sequence (?>). If the code has the correct syntax, it executes exactly as the programmer specified.
PHP is a language that can be embedded directly into the HTML code of pages, which, in turn, will be correctly processed by the PHP interpreter. We can use PHP to write CGI scripts and get relieved of a lot of awkward(inconvenient) text output statements. We can use PHP to generate HTML documents without a lot of external script calls.
The wide variety of PHP functions will save you the trouble of writing multi-line user-defined functions in C or Pascal.

Efficiency

Efficiency is of the utmost importance when programming for multi-user environments, which include the web.
A very important advantage of PHP lies in its “engine”. The PHP “engine” is neither a compiler (translate full or complete program)  nor an interpreter (translate only one statement at a time). It is a broadcast interpreter. Such a device “engine” PHP allows you to process scripts at a fairly high speed.
By some estimates, most PHP scripts (especially not very large ones) are processed faster than their counterparts written in Perl. However, no matter what PHP developers do, compiled executables will run much faster – by tens, and sometimes hundreds of times. But the performance of PHP is quite sufficient to create quite serious web applications. Details about the device and characteristics of the PHP “engine” can be found here.

Safety

PHP provides developers and administrators with flexible and powerful security tools that roughly fall into two categories: system-level tools and application-level tools.

1. System-level security features

PHP implements security mechanisms managed by administrators; When configured correctly, PHP provides maximum freedom of action and security. PHP can run in the so-called safe mode, which limits the ability of users to use PHP in a number of important ways. For example, you can limit the maximum execution time and memory usage (uncontrolled memory consumption negatively affects server performance). Similar to CGI-bin, the administrator can also set restrictions on the directories where the user can view and execute PHP scripts, and use PHP scripts to view sensitive information on the server (such as the passwd file).

2. Application layer security

PHP comes with a number of strong encryption mechanisms as standard. PHP is also compatible with many third-party applications(a program written by someone, not the manufacturer), making it easy to integrate with secure e-commerce technologies. Another advantage is that PHP scripts cannot be viewed in a browser because the script is compiled before it is sent to the user. PHP’s implementation on the server-side prevents non-trivial scripts from being stolen by users with even enough knowledge to execute the View Source command.

Details about PHP security can be found here

Flexibility

Since PHP is an embedded(have its own syntax) language, it is especially flexible in relation to the needs of the developer. While PHP is generally recommended to be used in conjunction with HTML, it integrates just as well into JavaScript, WML, XML, and other languages. In addition, well-structured PHP applications are easy to expand as needed (this applies to all major programming languages, by the way).

There are no browser dependency issues either, as PHP scripts are completely compiled on the server-side before they are sent to the client. In fact, PHP scripts can be delivered to any browser-enabled device, including cell phones, electronic notebooks, pagers, and laptops, not to mention traditional PCs. Helper programmers can run PHP in command line mode.

Since PHP does not contain code that targets a specific web server, users are not limited to certain servers (perhaps unfamiliar to them). The servers(PHP runs on all servers like Apache, Microsoft IIS ) run on different platforms, PHP is generally platform-independent and exists on platforms such as UNIX, Solaris, FreeBSD, and Windows 95/98/NT/2000/XP/2003.

Finally, PHP tools allow the programmer to work with external components such as Enterprise Java Beans(Server-side component) or Win32 COM objects(technology for Microsoft’s OLE, ActiveX). Thanks to these new capabilities, PHP takes its rightful place among modern technologies and ensures that projects scale up to the required limits.

Free distribution

The Open Source strategy, and the distribution of source codes to the masses, have undoubtedly had a beneficial effect on many projects, primarily Linux, although the success of the Apache project has strongly reinforced the positions of Open Source supporters. This also applies to the history of PHP, as the support of users from all over the world has been a very important factor in the development of the PHP project.

The adoption of the Open Source strategy and free distribution of PHP source code has done an invaluable service to users. In addition, PHP’s responsive user community is a kind of “collective help desk,” and even the most difficult questions can be answered in popular online forums.