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Defining functions

From: PHP with MySQL Essential Training

Video: Defining functions

A function is code that preforms a specific task, which is then packaged up into a single unit that can then be called upon whenever that task is needed. An example of a function that we've already seen and used would be in array. Any time we want PHP to whether an element is in an array we can call the in array function. We provide the function data to work with in the form of arguments. And the function returns output to us at the end. These are common feature functions, providing them input and getting back output. Up until now, we've been looking at the built-in functions that PHP provides, but we aren't limited to just these functions.

Defining functions

A function is code that preforms a specific task, which is then packaged up into a single unit that can then be called upon whenever that task is needed. An example of a function that we've already seen and used would be in array. Any time we want PHP to whether an element is in an array we can call the in array function. We provide the function data to work with in the form of arguments. And the function returns output to us at the end. These are common feature functions, providing them input and getting back output. Up until now, we've been looking at the built-in functions that PHP provides, but we aren't limited to just these functions.

We can also define our own. In this chapter, we'll learn how to craft and use custom functions. And we'll start by learning the syntax that's used to define them. You define a function by first saying, function, and then a space. And then the name that you want to give this function. It's very much like the variable names that we assign. And the rules are very similar. We can have letters, numbers, underscores, and dashes. You can't have any spaces, though. And it must start with either a letter or an underscore. Unlike variable names though, function names are case insensitive. So, with a variable, if we had capital var, and lower case var. Those were two different variable names.

But if we have a capital function and a lowercase function, they're going to be the exact same thing. However, it's really bad programming practice to start switching case around. So, whatever you define them as, that's how you're going to want to always call them as well. Pick one style and stick with it. For me, I like the all lowercase with underscores style. And that's what I'll be using. After we have the name, then we have parentheses with our argument list. These are the arguments that the function is going to accept. In this example, I've just got arg1 and arg2 separated by a comma between them.

Both of these are going to be variable names so they're going to have the dollar sign in front of them. We can have more than two, you just put a comma and have arg3, arg4 and so on. You'll probably want to use a better name for yours that will actually describe what you're expecting to get in. So, if you're looking for an array, the name of it might be array. And then, we've got our curly braces after that and inside that is the meat of the function. Its all of that code that we're wrapping up inside this function. This is the code that we want to reuse and be able to call from a lot of different places. This code tells PHP take this block of code inside the curly braces and keep it around.

File it under the name. And when I call for that name, take the input that I send run the code and give me back the output. Let's try an example. To start with, I will open up basic .html, and I will do Save As and we are going to call this functions_defining.php. And I'll give the title Functions Defining. Alright, so let's write our first functions in our PHP tags. To write our first function, we're first going to use the keyword function.

Followed by the name, and I'm going to call this one say_hello, and then the arguments. Now, there's not going to be any arguments to this one. But I'm still going to put the empty parentheses there so that it's clear that that's where the arguments would go. And then, curly braces. And then, whatever code we want to package up. Whatever we want to happen when we call this function. And I'm just going to have it echo the classic, Hello World, exclamation point. Put a br tag, and a semicolon at the end, okay? So there I have, I've defined my first function.

It is not something that pre-existed. This is something that I'm defining. Let's go to Firefox and take a look. So, I'll open up Firefox. And it's going to be localhost, and then for me, I'm going to use Kevin Scoglund sandbox and then functions_defining.php, okay? I didn't get anything back. Now, I am seeing the right page. And you can tell that because it says up here at the top, Functions Defining, right? So, that is the page that I'm on. And we can actually go to our Web Developer Tools. And we can view the Page Source.

And we can see that we got something. But there's nothing in there. Nothing gets output when you define your function. We have to then call the function. So, what we're saying to PHP is take this code and store it for later, because I may call on it. But we haven't done anything, we haven't called it. It just simply set it aside and assigned it the same way as if we assigned a variable a value. We just assigned a block of code to the say hello function. Now, in order to call it, we need to say hello. Let's try it now. Save that, let's go back over here.

And sure enough, Hello World pops up. Now, the parentheses here are not optional. If we are calling a function, we want to make it clear that it is a function. If we take it away, then PHP sees this as something different. Over here, just reload it so you can see the difference. You can see that we get a notice saying that we are using an undefined constant, say hello. And that's not what we want at all. We were trying to call our function. So, when we call our functions, even if there's no arguments. We're going to use just two parentheses afterwards. So, the correct way to say this is that we are calling the function.

We're calling the say_hello function. Alright, now let's try this with our first argument. Let's come down here and let's define another function, but this one is going to be say_hello_to, underscore to. And then, we'll put an argument in here and that will just be word, alright? So that's our first argument, is word. And then, we have the ability to use that argument inside our function. So for example, I can now have dollar sign word, right? See how that works? So this is now accessible to me inside me.

Just like my loop local variables are accessible inside of loop, well here. The function arguments are accessible inside the function. So let's let's try that out now. We're going to call it with say, hello_to, and we need to provide a word. So let's just start with world, right? Let's go back, save it. Reload the page and there you go. See we get the second time say Hello World. Now, why use arguments? Well, because we have flexibility then. That's the nice thing about arguments is we can reuse that code so we can have it do something slightly different based on the input that we pass in.

So, say hello to everyone. And that returns hello everyone. Now this is a super simple example but regardless of what task we're trying to perform. What we want is to take input and base don that input, get back different output. We don't want the same output every single time. Now, of course, we can't call a function that doesn't exist. Hello loudly. Let's try that. Come back over here, reload it. You'll see that you get fatal call to undefined function on say hello loudly. It's undefined, we didn't define it like we did these other ones. Now, it may surprise you to know, though, that we can actually take this and let's do say Hello Loudly.

And we'll just make this, Hello World, in all caps. Let's go back and try this now. Reload it and it worked but notice I defined it after I called it, right? It doesn't do it sequentially. In php3, it did have to precede calling it, but php4 and 5 actually pre-process the page to find all of the functions. First, now it's still a good programming practice to put them before calling them. Because someone looking at your code, is more likely to be reading from the top down.

And to be surprised that they haven't seen it defined. So, don't get into the habit of defining it later. But it's not going to cause you a problem if you do accidentally put a function after calling it. It's still going to be available to you. Now once we've defined a function, though. We can't redefine it. Just do another say_hello_loudly here. And I'll just change this to we can't redefine a function. Let's just come over her and try that out. And you'll see fatal error, cannot re-declare say_hello_loudly.

Previously declared at this spot. It tells us where it's located. So, we can't do that. So, I'm just going to comment that out so that won't cause our page to break anymore. Now, you can define functions inside other functions inside if statements, inside loops. But this is really advanced usage and can lead to many problems and unexpected results for beginners. So for now, be sure that you're always defining your functions at the quote "root" of your PHP page, not inside any other PHP structure. Now that we understand the fundamentals of defining functions, let's explore how to use them a little further in the next movie.

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This video is part of

Image for PHP with MySQL Essential Training
PHP with MySQL Essential Training

131 video lessons · 33337 viewers

Kevin Skoglund
Author

 
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  1. 4m 8s
    1. Welcome
      1m 0s
    2. Using the exercise files
      3m 8s
  2. 15m 6s
    1. What is PHP?
      3m 52s
    2. The history of PHP
      2m 51s
    3. Why choose PHP?
      4m 10s
    4. Installation overview
      4m 13s
  3. 54m 53s
    1. Overview
      2m 33s
    2. Working with Apache Web Server
      6m 56s
    3. Changing the document root
      7m 24s
    4. Enabling PHP
      6m 16s
    5. Upgrading PHP
      3m 30s
    6. Configuring PHP
      10m 3s
    7. Installing MySQL
      5m 46s
    8. Configuring MySQL
      7m 24s
    9. Text editor
      5m 1s
  4. 31m 25s
    1. Overview
      3m 27s
    2. Installing WampServer
      5m 46s
    3. Finding the document root
      2m 24s
    4. Configuring PHP
      8m 12s
    5. Configuring MySQL
      5m 45s
    6. Text editor
      5m 51s
  5. 19m 12s
    1. Embedding PHP code on a page
      6m 43s
    2. Outputting dynamic text
      5m 55s
    3. The operational trail
      2m 27s
    4. Inserting code comments
      4m 7s
  6. 1h 18m
    1. Variables
      7m 50s
    2. Strings
      4m 38s
    3. String functions
      8m 54s
    4. Numbers part one: Integers
      6m 27s
    5. Numbers part two: Floating points
      5m 25s
    6. Arrays
      10m 0s
    7. Associative arrays
      6m 37s
    8. Array functions
      6m 33s
    9. Booleans
      3m 50s
    10. NULL and empty
      5m 15s
    11. Type juggling and casting
      8m 27s
    12. Constants
      4m 43s
  7. 27m 37s
    1. If statements
      6m 0s
    2. Else and elseif statements
      4m 16s
    3. Logical operators
      7m 30s
    4. Switch statements
      9m 51s
  8. 42m 15s
    1. While loops
      8m 41s
    2. For loops
      5m 59s
    3. Foreach loops
      8m 16s
    4. Continue
      8m 28s
    5. Break
      4m 8s
    6. Understanding array pointers
      6m 43s
  9. 37m 25s
    1. Defining functions
      8m 25s
    2. Function arguments
      5m 32s
    3. Returning values from a function
      7m 33s
    4. Multiple return values
      4m 53s
    5. Scope and global variables
      6m 2s
    6. Setting default argument values
      5m 0s
  10. 20m 18s
    1. Common problems
      3m 47s
    2. Warnings and errors
      8m 36s
    3. Debugging and troubleshooting
      7m 55s
  11. 57m 57s
    1. Links and URLs
      5m 33s
    2. Using GET values
      5m 35s
    3. Encoding GET values
      8m 41s
    4. Encoding for HTML
      9m 26s
    5. Including and requiring files
      7m 40s
    6. Modifying headers
      6m 45s
    7. Page redirection
      6m 43s
    8. Output buffering
      7m 34s
  12. 1h 3m
    1. Building forms
      7m 28s
    2. Detecting form submissions
      5m 59s
    3. Single-page form processing
      7m 57s
    4. Validating form values
      10m 40s
    5. Problems with validation logic
      9m 54s
    6. Displaying validation errors
      7m 23s
    7. Custom validation functions
      6m 28s
    8. Single-page form with validations
      7m 25s
  13. 28m 5s
    1. Working with cookies
      2m 49s
    2. Setting cookie values
      5m 55s
    3. Reading cookie values
      6m 1s
    4. Unsetting cookie values
      4m 51s
    5. Working with sessions
      8m 29s
  14. 48m 39s
    1. MySQL introduction
      6m 43s
    2. Creating a database
      7m 41s
    3. Creating a database table
      7m 42s
    4. CRUD in MySQL
      5m 48s
    5. Populating a MySQL database
      7m 32s
    6. Relational database tables
      6m 40s
    7. Populating the relational table
      6m 33s
  15. 56m 4s
    1. Database APIs in PHP
      4m 51s
    2. Connecting to MySQL with PHP
      7m 45s
    3. Retrieving data from MySQL
      8m 47s
    4. Working with retrieved data
      6m 12s
    5. Creating records with PHP
      6m 58s
    6. Updating and deleting records with PHP
      9m 6s
    7. SQL injection
      3m 5s
    8. Escaping strings for MySQL
      6m 45s
    9. Introducing prepared statements
      2m 35s
  16. 35m 58s
    1. Blueprinting the application
      7m 19s
    2. Building the CMS database
      5m 14s
    3. Establishing your work area
      4m 38s
    4. Creating and styling the first page
      4m 22s
    5. Making page assets reusable
      6m 36s
    6. Connecting the application to the database
      7m 49s
  17. 32m 49s
    1. Adding pages to the navigation subjects
      5m 58s
    2. Refactoring the navigation
      6m 7s
    3. Selecting pages from the navigation
      6m 2s
    4. Highlighting the current page
      5m 26s
    5. Moving the navigation to a function
      9m 16s
  18. 1h 45m
    1. Finding a subject in the database
      9m 48s
    2. Refactoring the page selection
      10m 52s
    3. Creating a new subject form
      6m 55s
    4. Processing form values and adding subjects
      11m 20s
    5. Passing data in the session
      9m 16s
    6. Validating form values
      9m 40s
    7. Creating an edit subject form
      8m 30s
    8. Using single-page submission
      7m 44s
    9. Deleting a subject
      9m 44s
    10. Cleaning up
      10m 37s
    11. Assignment: Pages CRUD
      4m 30s
    12. Assignment results: Pages CRUD
      6m 10s
  19. 39m 26s
    1. The public appearance
      8m 52s
    2. Using a context for conditional code
      11m 37s
    3. Adding a default subject behavior
      6m 9s
    4. The public content area
      5m 51s
    5. Protecting page visibility
      6m 57s
  20. 1h 3m
    1. User authentication overview
      4m 3s
    2. Admin CRUD
      8m 41s
    3. Encrypting passwords
      7m 26s
    4. Salting passwords
      5m 42s
    5. Adding password encryption to CMS
      11m 54s
    6. New PHP password functions
      3m 13s
    7. Creating a login system
      11m 28s
    8. Checking for authorization
      5m 48s
    9. Creating a logout page
      5m 40s
  21. 2m 4s
    1. Next steps
      2m 4s

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