Purpose of PHP parameters - php

Question

I am trying to grasp the concept of PHP functions. I know how to create one.
function functionName()
{
//code to be executed;
}
I also know how to call a function. I am just a little confused as to what a parameter is for. I have read the php manual and w3schools.com's tutorial. From my understanding, you need a parameter to pass a value to the function? If that is correct why not just create it within the function? Why use a parameter?
Like this:
<?php
function num()
{
$s=14;
echo $s;
}
num();
?>
I know you can do:
<?php
function num($s=14)
{
echo $s;
}
num();
?>
or:
<?php
function num($s)
{
echo $s;
}
num($s=14);
?>
Could someone give me a real application of using a parameter, for say maybe a user based dynamic content website? I think it would help me understand it better.

Answer 1

Passing a parameter allows you to use one function numerous times. For example:
If you wanted to write ONE function that sent mail - you could pass the following parameters:
$to = $_POST['to'];
$from = $_POST['from'];
$subject = $_POST['subject'];
Then, in your function:
function sendmail($to, $from, $subject){
//code to be executed
}
Now you can reuse your send function at various points in your web app.

Answer 2

Here is an example, say you have numbers representing colors (this is common in storing data in a database) and you want to output what number represent's what color.
Say you had to do this a hundrend times for a hundred numbers.
You'd get pretty tired writing 100 if statments 100 times.
Here is a function example...
function colorType($type) {
if ($type == 1) {
return "Green";
}
elseif ($type == 2) {
return "Blue";
}
elseif ($type == 3) {
return "Red";
}
// etc
}
echo colorType(1) . "<br>"; // Green
echo colorType(2) . "<br>"; // Blue
echo colorType(3) . "<br>"; // Red

Answer 3

A function does something, and gives a result. It may accept parameters to arrive at that result, it may not. The simple calculator, as aforementioned, is a good one.
The easiest way to understand functions and parameters is to just read the PHP manual—most of the functions in the core PHP language take parameters of some sort. These functions are no different to the functions you write.

Answer 4

Let's assume you want to create a function that will allow people to sum numbers, you can't write needed variables in functions because you want others to input it and your function shows output:
function add($num1, $num2){
return $num1 + $num2;
}
Now anyone can call/use your function to sum numbers:
echo add(5,1); // 6
echo add(2,1); // 3
echo add(15,1); // 16
That's the most simplest example one can give to explain why you need parameters :)

Answer 5

When you specify function name($var=VALUE), you are setting a default.
function doit($s=14) {
return $s + 5;
}
doit(); // returns 19
doit(3); // returns 8

Answer 6

it makes your functions flexible to be reused in various situations, otherwise you would have to write many functions, one for each scenario. this is not only tedious, but becomes a nightmare if you have to fix something in those functions. instead of fixing it in one place, you would have to fix it in many places. you basically never want to have to copy paste code you have already written, instead you use arguments to make one set of the code flexible enough to handle each situation.

Answer 7

Paramaters allow your function to see the value of variables that exist outside of itself.
For example:
function F_to_C($temp) {
$temp = ($temp - 32) / 1.8;
return $temp;
}
$temperature = 32;
$new_temperature = F_to_C($temperature); // 0
echo $temperature;
$temperature2 = F_to_C(212); // 100
echo $temperature2;
Here we take $temperature, which we define in the code, but could be user input as from a form, and then send it to the function F_to_C. This allows us to convert it to Celsius, so we can then display it thereafter. In the next section, we then re-use the function to convert the boiling point, which is sent directly this time as the value 212. If we had embedded $temperature = 32 in the function the first time, then we would still get 0 as a result. However since we're using parameters, we instead get 100 back, because it's processing the value we specified when we invoked the function.