幾乎所有的編程語言的主要特點之一是可按需創建可重用代碼的能力。在PHP中,這是通過創建用戶定義的函數。在這篇文章中,我將向您展示在PHP中如何定義和使用函數。
創建函數
- 可以定義函數在PHP腳本中幾乎任何地方。一個PHP腳本,這包括全局作用域,在其他函數或在一個類中。
- 使用關鍵字function 和 函數名稱定義函數。
- A function may have 0 or more arguments, indicated between parentheses after the function name
- The code body of a function appears between an opening and closing brace
- A valid function name begins with a letter or underscore, following by any number of letters, numbers or underscores
- Unlike variable names, function names are case-insensitive
- A function is called using the function name and parentheses (which may or may not include arguments between the parentheses)
The following PHP code defines two basic functions: one with an argument and one without. It also shows how to call both functions.
{
echo 'First';
}
function second($name)
{
echo 'Second: '.$name;
}
// call the first function
first();
// call the second function
second('some argument');
## Output:
## ------------------------------
## First
## Second: some argument
?>
- Unlike many other languages, functions do not need to be declared or defined before they are called. Exceptions to this rule: if a function is defined within another function or conditionally defined you can only call that function after it has been created.
- All functions are created in the global scope. This means even if a function b() was defined in function a()you can still call b() from outside of a()
- Recursion is supported in PHP, but should be avoided if possible (the PHP manual states a function with over 100-200 levels may cause issues)
Function Arguments
Data can be passed to PHP functions using arguments. The ability to have default values, variable argument lists and type definitions make functions very powerful.
- Variables can be passed either by value (a fresh copy of the value inside the function) or by reference (the same variable being used inside and outside of the function)
- Objects are automatically passed by reference
- Other types can be passed by reference be preceding the argument name with &
The following code demonstrates the difference between passing by value and passing by reference. The $b value is passed by reference. When it is output at the end you can see its value has been modified.
$byValue++;
$byReference++;
echo sprintf("Inside: %d, %d\n", $byValue, $byReference);
}
$a = 5;
$b = 10;
echo sprintf("Before: %d, %d\n", $a, $b);
sampleFunc($a, $b);
echo sprintf("After: %d, %d\n", $a, $b);
## Output:
## ------------------------------
## Before: 5, 10
## Inside: 6, 11
## After: 5, 11
?>
- You can specify default values for arguments. If the argument is omitted from the function call the default is used
- The default value must be a constant expression (such as a string, integer, or NULL)
- You can have multiple arguments with default values, but these should always appear after non-optional arguments
The following code demonstrates how default values can be used:
{
echo sprintf("%s is %s\n", $name, $size);
}
nameAndSize("John");
nameAndSize("Paul", null);
nameAndSize("George", "tall");
## Output:
## ------------------------------
## John is small
## Paul is
## George is tall
?>
Variable-Length Argument Lists
In addition to supporting default values for function arguments, PHP supports variable-length argument lists with functions such as func_get_args().
Regardless of what the function definition dictates, you can retrieve every single value passed to a function usingfunc_get_args(). This function returns an array, where each element corresponds to a single argument. This is useful it you want your function to accept any number of arguments.
The following function demonstrates how you may achieve this. In this example the function definition still requires at least one value to be passed (even though it's not accessed directly).
{
foreach(func_get_args() as $arg){
echo $arg."
";
}
}
outputUsers("John", "Paul", "George", "Ringo");
## Output:
## ------------------------------
## John
## Paul
## George
## Ringo
?>
Returning Values
- You can return a value from a function using the return() keyword
- If there's no return() executed then NULL is the default return value
- It is typically good practice to use as a few returns in a function as possible (only one if possible)
- You can only return a single value from a function, but there are two ways around this
- First way: return a more complex value such as an array or an object which encapsulates multiple values
- Second way: use by-reference arguments and update those values in your function
The following listing demonstrates how you can use return values:
{
return 1234;
}
function bar()
{
return array(1234, 5678);
}
$num = foo();
$arr = bar();
?>
The following listing demonstrates more complex usage of return values in conjunction with arguments passed by reference.
* Try to write a value to the passed variable
*
* @param string &$buffer The variable to write to
* @return bool True if successful, false if not
*/
function writeToBuffer(&$buffer){
$ret = false;
if(strlen($buffer) > 0){
$buffer .= "some text";
$ret = true;
}
return $ret;
}
$buffer = "adsf";
if(writeToBuffer($buffer)){
echo sprintf("Success: %s\n", $buffer);
}else{
echo "Failure\n";
}
## Output:
## ------------------------------
## Success: adsfsome text
?>
Anonymous Functions
Since PHP 5.3.0, PHP has supported anonymous functions (or closures). These are primarily useful as a callback for other functions.
The following listing (taken from PHP manual) demonstrates using a closure directly as a callback:
echo preg_replace_callback("~-([a-z])~", function($match){
return strtoupper($match[1]);
}, $value);
// outputs helloWorld
?>
You can also assign a closure directly to a PHP variable:
New Features in PHP (from v5.4)
- Function array dereferencing has been added, e.g. foo()[0].
- Closures now support $this.
Array dereferencing allows you to do the following.
{
return array("a" => "apple", "b" => "banana");
}
echo fruit()["a"];
// echoes "apple"
// This roughly translates to using the following code in a current version of PHP.
function fruit()
{
return array("a" => "apple", "b" => "banana");
}
$fruits = fruit();
echo $fruits["a"];
// echoes "apple"
?>