Functions are a technique for making a script easier to manage, and to save memory space. Actions that are executed more than once within a programme can be efficiently defined and executed by a function, whose name is sugestive of its purpose.
Built-in functions (e.g. isset, echo, order, sort, include) cover to a large degree the needs of a PHP script. However, custom functions can be defined simply to aid the effectiveness of a programme. They are defined by the following syntax:
Function names may include any combination of alphanumeric characters, but the first character must be a letter or the underscore (_).
Custom functions cannot have the same name as built-in functions. Function names are case insensitive, so
newFunction() is the same function as
new_Function() will not call it.
Functions must be defined before they are called, or an
undefined function error will result. It is therefore logical to define functions near the top of a page of script.
Both built-in and custom functions can take arguments (parameters). An argument is information defined by a constant or a variable, which is imported into a function from the script that calls the function.
To call the function with the arguments predefined, the following code appears somewhere in the script:
Arguments may also have a preset default value:
The default argument is now optional when calling the function. If the value is not passed to the function, it will use the default.
A function can return a value:
The scope of a variable is the 'realm' where it has a value and can be accessed. A superglobal variable is always valid. Variables defined on a page are generally valid for that page, but not on another. To pass a variable to another page, the
$_GET method can be used.
Where care must be given is variable scope outside and inside functions. Unless specified with
global $variable;, a variable defined outside a function is not accessible from within the function, and a variable assigned a value within a function does not have this value outside the function.
str_replace() will replace the first string (X) with the second (Y), in the third element ($oldvar).
In this example, the URL brought to a page by the GET function, has underscores (this is because URL uses
%20 in place of spaces, so an underscore is used in the URL title) which need to be replaced by spaces before the
$topic variable is used in the page itself.
mysqli_real_escape_string PHP function escapes special characters in the escapestring so that the modified escapestring may by used in an SQL statement. The escaped characters are: NUL (ASCII 0), \n, \r, \, ', ", and Control-Z.
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