5.4.0 and later it is always available. The purpose of all these delimiters is to separate PHP code from non-PHP content, such as JavaScript code or HTML markup. So the shortest "Hello, World!" program written in PHP is: <?='Hello, World!'; The first form of delimiters, <?php and ?>, in XHTML and other XML documents, creates correctly formed XML processing instructions. This means that the resulting mixture of PHP code and other markup in the server-side file is itself well-formed XML. Variables are prefixed with a dollar symbol, and a type does not need to be specified in advance. PHP 5 introduced type declarations that allows functions to force their parameters to be objects of a specific class, arrays, interfaces or callback functions. However, before PHP 7, type declarations could not be used with scalar types such as integer or string. Below is an example of how PHP variables are declared and initialized. <?php $name = 'John'; // variable of string type being declared and initialized $age = 18; // variable of integer type being declared and initialized $height = 5.3; // variable of double type being declared and initialized echo $name . ' is ' . $height . "m tall\n"; // concatenating variables and strings echo "$name is $age years old."; // interpolating variables to string ?> Unlike function and class names, variable names are case sensitive. Both double-quoted ("") and heredoc strings provide the ability to interpolate a variable's value into the string. PHP treats newlines as whitespace in the manner of a free-form language, and statements are terminated by a semicolon. PHP has three types of comment syntax: /* */ marks block and inline comments; // or # are used for one-line comments. The echo statement is one of several facilities PHP provides to output text. In terms of keywords and language syntax, PHP is similar to the C style syntax. if conditions, for and while loops, and function returns are similar in syntax to languages such as C, C++, C#, Java and Perl. Data types PHP is loosely typed. It stores integers in a platform-dependent range, either as a 32, 64 or 128-bit signed integer equivalent to the C-language long type. Unsigned integers are converted to signed values in certain situations, which is different behavior to many other programming languages. Integer variables can be assigned using decimal (positive and negative), octal, hexadecimal, and binary notations. Floating point