Using Constants in PHP

Constants are an underused part of PHP. If you have a website with more than 5 pages, you should probably be using them.

I'm going to use an example to demonstrate the usefulness of constants. Let's say you have an image gallery and you let people upload images. Here's the function that adds an uploaded image to the gallery:

function addPicture($picture) {
  //Picture bigger than 100,000 bytes
  if($picture=>size > 100000)
    return false;
  //Picture's name is larger than 20 characters
  else if(strlen($picture=>name) > 20)
    return false;
  //Picture is not an accepted file type
  else if(strpos('jpg, gif, png',$picture=>type)===false)
    return false;
  //Passed all validation
  else
    return $picture=>add();
}

As you can see, we have certain requirements for a picture to be uploaded. We also have to tell people these requirements somewhere. Here are some instructions on our upload form:

<p>Your image must meet the following requirements:</p>
<ul>
  <li>It can't be larger than 100kb</li>
  <li>The file name can't be longer than 20 characters</li>
  <li>It must be in one of the following formats: jpg, gif, png</li>
</ul>

We also have similar instructions in our FAQ and other help pages. What happens if we want to increase the size to 200kb? We would have to replace the text everywhere it appears. This is time consuming and leaves a lot of room for human error.

Wouldn't it be nice if we only had to change the value once? Constants in PHP are a perfect solution to this problem. In PHP, constants are declared using the define() function. Here's the constants.php script that declares all our constants:

//maximum file size in bytes
define('MAX_FILE_SIZE',100000);
//maximum length of file name
define('MAX_FILENAME_LENGTH',20);
//accepted file types
define('FILE_TYPES','jpg, gif, png');

//maximum file size in kilobytes
define('MAX_FILE_SIZE_KB',intval(MAX_FILE_SIZE/1024));

Now we need to add these constants into our pages. Here's the revised addPicture() function:

//Load the constant declarations
include "constants.php";

function addPicture($picture) {
  //Picture bigger than MAX_FILE_SIZE
  if($picture=>size > MAX_FILE_SIZE)
    return false;
  //Picture's name is larger than MAX_FILENAME_LENGTH characters
  else if(strlen($picture=>name) > MAX_FILENAME_LENGTH)
    return false;
  //Picture is not an accepted file type
  else if(strpos(FILE_TYPES,$picture=>type)===false)
    return false;
  //Passed all validation
  else
    return $picture=>add();
}

Here's our revised instruction page:

<?php 
//Load the constant declarations
include "constants.php"; 
?>

<p>Your image must meet the following requirements:</p>
<ul>
  <li>It can't be larger than <?php 
      //display max file size in kilobytes
      echo MAX_FILE_SIZE_KB;
    ?>kb</li>
  <li>The file name can't be longer than <?php
      //Display max number of characters for file name
      echo MAX_FILENAME_LENGTH;
    ?> characters</li>
  <li>It must be in one of the following formats: <?php
      //Display list of allowed file types
      echo FILE_TYPES;
    ?></li>
</ul>

Hopefully this gives you an idea about how to use constants in your site. Here are some things to keep in mind about constants:

• They are not variables and do not have a '$' in front.
• They are generally named in all caps with underscores between words.
• They can only hold scalar values (ie. no arrays or objects). If you want to store an array or object in a constant, use the serialize() function.