Sams Teach Yourself PHP, MySQL and Apache All in One (2012)

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Listing 7.14 defines two functions, tagWrap() (line 2) and underline() (line 12). The tagWrap() function accepts three strings: a tag, the text to format, and an optional function name. It returns a formatted string. The underline() function requires a single argument—the text to be formatted—and returns the text wrapped in <span> tags with appropriate style attributes.

When you first call tagWrap() on line 16, you pass it the string strong and the string "make me bold". Because you haven’t passed a value for the function argument, the default value (an empty string) is used. Line 4 checks whether the $func variable contains characters, and, if it is not empty,function_exists() is called to check for a function by that name. Of course, in this case, the $func variable is empty, so the $txt variable is wrapped in <strong> tags in the else clause on lines 7 and 8 and the result is returned.

The code calls tagWrap() on line 17 with the string 'em', some text, and a third argument: "underline". function_exists() finds a function called underline() (line 12), so it calls this function and passes the $txt argument variable to it before any further formatting is done. The result is an italicized, underlined string.

Finally, on line 18, the code calls tagWrap(), which wraps text in quotation entities. It is quicker to simply add the entities to the text to be transformed ourselves, but this example serves to illustrate the point that function_exists() works as well on anonymous functions as it does on strings representing function names.

Put these lines into a text file called exists.php and place this file in your web server document root. When you access this script through your web browser, it should look like Figure 7.8.

Figure 7.8 Output of exists.php.

Summary

This chapter taught you about functions and how to deploy them. You learned how to define and pass arguments to a function, how to use the global and static statements, how to pass references to functions, and how to create default values for function arguments. You also learned to test for the existence of functions.

Q&A

Q. Can I include a function call within a double- or single-quoted string, as I can with a variable?

A. No. You must call functions outside quotation marks. However, you can break the string apart and place the function call between the parts of the string, using the concatenation operator to tie them together, as follows:

$newstring = "I purchased".numPurchase($somenum)." items.";

Q. What happens if I call a function that does not exist, or if I declare a function with a name already in use?

A. Calling a function that does not exist or declaring a function with the same name as another existing function causes the script to stop execution. Whether an error message displays in the browser depends on the error settings in your php.ini file.

Workshop

The workshop is designed to help you review what you’ve learned and begin putting your knowledge into practice.

Quiz

1. True or false: If a function doesn’t require an argument, you can omit the parentheses in the function call.

2. How do you return a value from a function?

3. What does the following code fragment print to the browser?

$number = 50;

function tenTimes() {
$number = $number * 10;
}

tenTimes();
echo $number;

4. What does the following code fragment print to the browser?

$number = 50;

function tenTimes() {
global $number;
$number = $number * 10;
}

tenTimes();
echo $number;

5. What does the following code fragment print to the browser?

$number = 50;

function tenTimes( &$n ) {
$n = $n * 10;
}

tenTimes( $number );
echo $number;

Answers

1. The statement is false. You must always include the parentheses in your function calls, whether or not you are passing arguments to the function.

2. You must use the return keyword.

3. It prints 50. The tenTimes() function has no access to the global $number variable. When it is called, it manipulates its own local $number variable.

4. It prints 500. This example uses the global statement, which gives the tenTimes() function access to the $number variable.

5. It prints 500. By adding the ampersand to the parameter variable $n, you ensure that this argument is passed by reference. $n and $number point to the same value, so any changes to $n are reflected when you access $number.

Activities

1. Review the syntax for creating a function that accepts arguments, manipulates those values, and returns a string.

2. Create a function that accepts four string variables and returns a string that contains an HTML table element, enclosing each of the variables in its own cell.