Web Database Applications with PHP and MySQL, 2nd Edition

Functions

A function is another concept that programming derived from mathematics. Some programming functions are direct implementations of common mathematical functions, such as sines and other trigonometric functions. (Naturally, these are not used much in PHP.) But you are sure to use functions related to strings, dates, and other everyday objects in your code. PHP has a large number of useful functions built in, and you can define your own functions as we describe later in this chapter.

Functions are called in PHP scripts and can often be used as expressions. For instance, the following example uses the strtoupper( ) function to change a string to uppercase:

$var = "A string";

print strtoupper($var); // prints "A STRING"

A function is followed by parentheses, which can contain zero or more parameters. This function accepts just one parameter. It can be summarized as follows:

string strtoupper(string subject)

The previous statement is called a prototype and is very useful for introducing functions. Prototypes are used throughout PHP documentation, and the following chapters of this book, when a function is described. The first word indicates what is returned by the function: in this case, its output is a string. The name of the function follows, and then a list of parameters within parentheses. Each parameter is described by a type and a parameter name—strtoupper( ) is defined with a single string parameter named subject. Names allow us to distinguish multiple parameters when we ...