Functions in PHP

In this part of the PHP programming tutorial, we talk about functions.

A function is a piece of code in a larger program. The function performs a specific task. The advantages of using functions are:

There are two basic types of functions. Built-in functions and user defined ones. The built-in functions are part of the PHP language. Examples are: phpinfo(), round() or abs(). The user defined functions are created by application programmers to cover their needs. They are created with the function keyword.

Defining functions

A function is created with the function keyword.

simple.php
<?php

function displayVersion() {

    echo "this is PHP " . phpversion();
    echo "\n";
}

displayVersion();

?>

The function keyword is followed by the function name with round brackets. The body of the function lies between the curly brackets. We say that we call a function. If we call a function, the statements inside the function body are executed.

displayVersion();

This line of the code calls the function.

$ php simple.php 
this is PHP 5.5.9-1ubuntu4.14

Here we see the outcome of the script.

The return keyword

The return keyword is used to return a value from the function. A function may or may not return a value.

returning.php
<?php

function maximum($x, $y) {
    
    if ($x > $y) { 
        
        return $x;
    } else {
        
        return $y;
    }
}

$a = 23;
$b = 32;

$val = maximum($a, $b);
echo "The max of $a and $b is $val \n";

?>

We have a maximum() function. It returns a max for two numbers. We could not name it max, because there is already a built-in max() function. This example was created for learning purposes; we would always prefer the built-in functions in our real-world programs.

if ($x > $y) { 
    
    return $x;
} else {
    
    return $y;
}

If the $x variable is greater than $y, we return $x. Otherwise we return $y.

$val = maximum($a, $b);

The value returned by the maximum() function is assigned to the $val variable.

echo "The max of $a and $b is $val \n";

We print the max value of the two numbers to the console.

Function arguments

Most functions accept arguments. Arguments are values that are sent to the function. The functions process the values and possibly return some outcome.

fahrenheit.php
<?php

function FTC($c) {
    
    return $c * 9/5 + 32;
}

echo FTC(100);
echo "\n";
echo FTC(0);
echo "\n";
echo FTC(30);
echo "\n";

?>

In our example, we convert Fahrenheit temperature to Celsius. The FTC() function accepts one argument $c, which is the Celsius temperature.

$ php fahrenheit.php 
212
32
86

Implicit values

The arguments in PHP functions may have implicit values. An implicit value is used if no value is provided.

implicit_value.php
<?php

function power($a, $b=2) {

    if ($b == 2) {
    
        return $a * $a;
    }

    $value = 1;

    for ($i = 0; $i < $b; $i++) {
    
        $value *= $a;
    }
    
    return $value;
}

$v1 = power(5);
$v2 = power(5, 4);

echo "5^2 is $v1 \n";
echo "5^4 is $v2 \n";

?>

Here we created a power function. The function has one argument with an implicit value. We can call the function with one or two arguments.

$ php implicit_value.php 
5^2 is 25 
5^4 is 625 

Variable number of arguments

A function may accept variable number of arguments. In other words, sometimes we do not know how many arguments will be passed to the function. The func_get_args() function returns an array comprising a function's argument list.

Since PHP 5.6 there is the ... operator available to create variadic functions.

variable_arguments1.php
<?php

function sum(...$nums) {
    
    $sum = 0;
    
    foreach ($nums as $n) {
        $sum += $n;
    }
    
    return $sum;
}

echo sum(1, 2, 3) . "\n";
echo sum(1, 2, 3, 4) . "\n";
echo sum(1, 2, 3, 4, 5) . "\n";

?>

We create a sum() method which can take variable number of arguments. The method calculates the sum of values passed to the method.

$sum = 0;
foreach ($args as $n) {
    $sum += $n;
}

return $sum;

We calculate and return the sum of the values.

echo sum(1, 2, 3) . "\n";
echo sum(1, 2, 3, 4) . "\n";
echo sum(1, 2, 3, 4, 5) . "\n";

We pass three, four, and five values to the sum() function.

$ php variable_arguments1.php 
6
10
15

This is the output.

variable_arguments2.php
<?php

function sum() {
    
    $args = func_get_args();
    
    $sum = 0;
    foreach ($args as $n) {
        $sum += $n;
    }
    
    return $sum;
}

echo sum(1, 2, 3) . "\n";
echo sum(1, 2, 3, 4) . "\n";
echo sum(1, 2, 3, 4, 5) . "\n";

?>

Now the same example is created with the func_get_args() function.

Static variables

A static variable is a variable that has been allocated statically, whose lifetime extends across the entire run of the program. The default local variables do not retain their value within consecutive calls of the function.

non_static.php
<?php

function nonstatic() {

    $value = 0;
    $value += 1;

    return $value;
}

nonstatic();
nonstatic();
nonstatic();
nonstatic();

echo nonstatic(), "\n";

?>

In the above example, we have a normal, non-static variable. We increment the variable each time the function is called. We call the function 5 times. However, non-static variables are initiated for each call of the function. After 5 function calls the $value equals to 2.

The static variables are initiated only once, when the function is first called. They retain their value afterwards.

static.php
<?php

function staticfun() {

    static $value = 0;
    $value += 1;

    return $value;
}

staticfun();
staticfun();
staticfun();
staticfun();

echo staticfun(), "\n";

?>

After 5 consecutive calls, the $value is equal to 5.

$ php nonstatic.php 
2
$ php static.php 
6

Anonymous functions

Anonymous functions do not have a name.

anonymous.php
<?php

$var = function() {
    
    echo "This is anonymous function\n";
};

$var();

?>

We assign a function body to a variable. It is possible to call the function only via this variable.

$var = function() {
    
    echo "This is anonymous function\n";
};

Notice the semicolon after the closing curly bracket. It is required because the construct is an assignment.

$ php anonymous.php 
This is anonymous function

This is the output of the example.

Passing arguments by value and by reference

PHP supports two ways of passing arguments to functions. The default way is passing arguments by value. When we pass arguments by value, the function works only with the copies of the values. This may lead to performance overheads when we work with large amounts of data.

When we pass values by reference, the function receives a reference to the actual value. The original values are affected when modified. This way of passing values is more time and space efficient. On the other hand, it is more error prone.

Which way of passing arguments should we use? It depends on the situation. Say we have a set of data, salaries of employees. If we want to compute some statistics of the data, we do not need to modify them. We pass by values. If we work with large amounts of data and the speed of computation is critical, we pass by reference. If we want to modify the data, e.g. do some reductions or raises to the salaries, we might pass by reference.

The following two examples cover both concepts.

swap1.php
<?php

function swap($a, $b) {

    $temp = $a;
    $a = $b;
    $b = $temp;
    echo "inside swap function:\n";
    echo "\$a is $a \n";
    echo "\$b is $b \n";
}

$a = 4;
$b = 7;

echo "outside swap function:\n";
echo "\$a is $a \n";
echo "\$b is $b \n";

swap($a, $b);

echo "outside swap function:\n";
echo "\$a is $a \n";
echo "\$b is $b \n";

?>

The swap function swaps the numbers between the $a and $b variables. The original variables are not affected.

$a = 4;
$b = 7;

At the beginning, these two variables are initiated.

swap($a, $b);

We call the swap() function. The function takes $a and $b variables as parameters.

$temp = $a;
$a = $b;
$b = $temp;

Inside the swap() function, we change the values. Note that the $a and $b variables are defined locally. They are valid only inside the swap() function.

$ php swap1.php 
outside swap function:
$a is 4 
$b is 7 
inside swap function:
$a is 7 
$b is 4 
outside swap function:
$a is 4 
$b is 7 

The output shows that the original variables were not affected.

The next code example passes values to the function by reference. The original variables are changed inside the swap() function.

swap2.php
<?php

function swap(&$a, &$b) {

    $temp = $a;
    $a = $b;
    $b = $temp;
    echo "Inside the swap function:\n";
    echo "\$a is $a \n";
    echo "\$b is $b \n";
}

$a = 4;
$b = 7;

echo "Outside the swap function:\n";
echo "\$a is $a \n";
echo "\$b is $b \n";

swap($a, $b);

echo "Outside the swap function:\n";
echo "\$a is $a \n";
echo "\$b is $b \n";

?>

We use the & character to pass values by reference.

function swap(&$a, &$b) {
...  
}

The example is almost identical to the previous one. Except for the ampersand characters.

$ php swap2.php 
Outside the swap function:
$a is 4 
$b is 7 
Inside the swap function:
$a is 7 
$b is 4 
Outside the swap function:
$a is 7 
$b is 4 

Here we see that the swap() function really changed the values of the variables.

Recursion

Recursion, in mathematics and computer science, is a method of defining functions in which the function being defined is applied within its own definition. In other words, a recursive function calls itself to do its job. Recursion is an alternative to iteration. Recursion is the main approach in functional languages.

A typical example is the calculation of the factorial.

recursion.php
<?php

function factorial($n) {

    if ($n==0) {
    
        return 1;
    } else {
    
        return $n * factorial($n - 1);
    }
}

echo factorial(4), "\n";
echo factorial(10), "\n";

?>

In this code example, we calculate the factorial of two numbers.

return $n * factorial($n - 1);

Inside the body of the factorial function, we call the factorial function with a modified argument. The function calls itself.

$ php recursion.php 
24
3628800

These are the results.

Global and local variables

Next we will talk about the scope of the variables in PHP. A scope is the range in which a variable can be referenced. When we work with functions, there are two basic scopes: the global and the local scope. The local scope is also called a function scope.

scope1.php
<?php

$value = 1;

function simple() {
    
    var_dump($value);
}

simple();

?>

A variable defined outside a function body cannot be referenced within a function.

$ php scope1.php 
PHP Notice:  Undefined variable: value in /home/janbodnar/prog/php/functions/scope1.php on line 7
NULL

The $value variable is NULL in the simple() function.

scope2.php
<?php

$value = 4;

function simple() {
    $value = 3;
    echo $value, "\n";
}

simple();

echo $value, "\n";

?>

In this example, we have two variables with the same name. They do not collide because they exist in different scopes.

$ php scope2.php 
3
4

The value is 3 inside the function and 4 outside the function.

In the next example, we will modify a value inside the function.

scope3.php
<?php

$value = 1;

function simple() {

    global $value;
    $value = 2;
}

echo $value, "\n";
simple();
echo $value, "\n";

?>

We use the global keyword to reference a variable defined outside the body of the function.

$ php scope3.php 
1
2

The $value was successfully modified inside the simple() function.

In this part of the PHP tutorial, we covered PHP functions.