1. PHP Math Functions

PHP provides nearly fifty functions that can perform various mathematical functions. Combined with mathematical operators, they can make your homework a lot easier!

The simplest math functions are those that only require a single parameter. This parameter is usually a number, which is accepted either as a variable or a string. The ceil() function, for example, will return a number rounded up to the nearest full number.

As you can see, the results of these functions can be echoed, stored in a variable, etc.

Some math functions accept multiple parameters. The rand() function, for example, generates a random number in between the two numbers that it is given. Multiple parameter are separated by commas.

If you refresh the page, you will see the above result change each time to a random number between 0 and 500.

Below is a list of almost twenty useful math functions, along with a brief description and required parameters.

A complete list of the math functions available can be found in the official PHP manual. Each function includes examples that will help you understand how to use it.

2. PHP String Functions

PHP provides nearly one hundred functions that can manipulate strings in various ways. Some (but not all!) of the functions that can be performed on strings are:

• Compare Two Strings
• Find a String In Another String
• Find Out How Many Instances of A String Occur In Another String
• Return Part of a String
• Replace Part of a String
• Trim Whitespace From The Ends of a String
• Make An Entire String Lowercase or Uppercase
• Convert All Applicable Characters In a String to HTML Entities, or Vice Versa
• Strip HTML & PHP Tags From Inside of a String

The simplest string functions are those that only require a single parameter. This parameter is the string itself, which is accepted either as a variable or as a single or double-quoted string. The strlen() function, for example, will return the length of the string that it is given.

As you can see, the results of these functions can be echoed, stored in a variable, etc.

Most functions accept multiple parameters. The strpos() function, for example, searches for occurrences of a string inside of another string, and requires two parameters in order to function. Multiple parameter are separated by commas.

The strpos() function will return a number indicating the position of the first match that it encounters. If you begin to do the counting yourself, you will see that the "c" for "cat" is the 17th character... so why did the function say 16? This is because most PHP functions begin counting with 0, so the position of the string's first character is 0, the second character is 1, etc.

(By the way, there were no cats left in the boat, because they were all copy cats.)

Below is a list of fifty useful string functions, along with a brief description and required parameters.

A complete list of the string functions available can be found in the official PHP manual. Each function includes examples that will help you understand how to use it.

3. PHP Variable Functions

PHP provides over thirty functions that can perform various functions related to variables. Most are beyond the scope of this tutorial, but several are useful, and even necessary to know.

Before we learn about the functions, let's take a look at the different types of data that can be stored in variables. Although PHP automatically assigns the data type of each variable based on the data that it contains, it will be good to know a little about each one.

Got all that? I didn't either when it was first introduced to me, and I've never really needed it, so let's move on.

Probably the most commonly used variable function is the isset() function, and, as the name implies, it can tell you whether or not a variable has been set.

The empty() function will determine whether or not a variable contains actual data. An empty string "", 0, NULL & FALSE are all considered empty variables.

The unset() function, confusingly stuck in the example above, will unset the variable so that it is not longer set. It can be reset later in the script, but until then it no longer exists.

Below are these and a few other variable functions that can be useful.

A complete list of the variable functions available can be found in the official PHP manual. Each function includes examples that will help you understand how to use it.

4. PHP Date & Time Functions

There are nearly fifty date and time functions, so for the purpose of this tutorial we will narrow them down to three.

Since we will be using the word "timestamp" throughout this page, we will begin by explaining what it is. A Unix Timestamp is the number of seconds that have passed since January 1, 1970. Currently, the timestamp is 1405345249, but since it changes every second, the number will increase if you refresh the page. (The timestamp reflects the time set on the server, and not on the client's computer.)

The PHP Time() Function

The time() function returns the current timestamp. The timestamp that you see in the previous paragraph is generated by a very small line of code: <?php echo time(); ?>

The time() function can also return a modified timestamp. You can add or subtract any number of seconds to the function in order to return the timestamp of a previous or upcoming date and time.

For example, to return a timestamp for next week, or to return a timestamp from last week, I can add or subtract 7 days by determining how many seconds are involved (7 days * 24 hours per day * 60 minutes in an hour * 60 seconds in an hour = number of second in 7 days).

The result will be:

The PHP Strtotime() Function

The strtotime() function accepts an English datetime description and turns it into a timestamp. It is a simple way to determine "next week" or "last monday" without using the time() function and a bunch of math.

Some examples are:

The PHP Date() Function

The date() function formats a timestamp so that it actually makes sense, such as 6:40 AM Monday, July 14, 2014.

The date() function accepts two arguments, according to the following syntax: date(format, timestamp);. The first argument is the format that you want the timestamp in. The second argument (optional) is the timestamp that you want formatted. If no timestamp is supplied, the current timestamp will be used.

PHP provides over thirty-five case-sensitive characters that are used to format the date and time. These characters are:

Using a combination of these characters and commas, periods, dashes, semicolons and backslashes, you can now format dates and times.

Calculating Past & Future Dates

It is very simple to compare two dates in php, to verify if one is past, present or future. Since all dates can be converted back into timestamps, all it takes is a simple comparison of the two timestamps, which are essentially numbers. If the date in question is a bigger number than the current date, than you know that your date is in the future, and if the date is smaller than the current date, you know that the date is already past.

5. PHP Includes & Requires

The include() function allows you to insert one file's contents into another file when that other file is executed.

In English, this means that if you want something like a header, footer, navigation, block of PHP code, etc. to appear on multiple pages of your website, there is a solution that does not involve typing it in every time. By putting this information into separate files and including those files whenever necessary, you can save on time, space and clutter by updating only a single file.

This function includes files with extensions such as .inc, .html, .php & .txt as needed. It includes data such as HTML markup, PHP code, and/or text. The best part is that if the included file contains PHP code, that PHP code can be used in (referenced by) the file that is doing the including.

(Example: If $variable is set in page1.php and page1.php is included in page2.php, then $variable can be used in page2.php below where the include() function includes page1.php.)

The include() function syntax is: include("path to file + name of file + file extension");

If the file cannot be found, or cannot be opened, etc. an error will be spit out on the page, and the remainder of the page will be executed normally.

The require() function is identical to the include() function, with the exception that, if the file cannot be found or the page cannot be opened, the require() function considers the error to be fatal and stops executing the remainder of the file. For this reason, require() is recommended instead of include(), because scripts should not continue after an error.

The include_once() and require_once() functions perform exactly the same as include() and require(), with the exception that, as their name implies, if the code from their files has already been included in the file, they will not be included again.

Guess what? You can name your included files anything you want, because your visitors will never see your included files, or even know that they exist!

I should probably mention that the include() and require() "functions" are not really functions, and that they are "language constructs" like the echo "function", but who cares, right?!

6. PHP Mail Function

The mail() function allows you to send emails, as long as the server has a fully-functional email system already set up.

This function accepts five different parameters, only four of which we will discuss here, and only three of which are required.

Listed in order of appearance, the parameters are: mail(to,subject,message,headers)

One of the most simple examples of the mail() function is:

The addition of headers allows you to specify who the email is from and who should be copied and/or blind copied on an email

HTML emails can be sent by specifying the appropriate content-type in the header.

7. PHP Regular Expressions

Regular expressions are patterns that define a list of characters. They can be used to search, replace and otherwise work with strings, but are most commonly used to validate forms, since forms allow users to enter in unknown data. We will learn here how to determine whether or not an email address uses the correct syntax.

In the past, PHP supported two methods of pattern matching (POSIX and PCRE). Now, PHP is dropping support for POSIX methods, leaving PCRE (Perl-Compatible Regular Expressions) our subject matter for this tutorial.

Regular expressions have their own set of functions:

There are two types of regular expressions. "Literal" characters are those that match themselves, and "metacharacters" are those that have another meaning. "Literal" characters can be a single character, a word, a phrase, etc. that is taken literally, at face-value. Using the preg_match() function, let's look at an example of a literal regular expression. (Note: Literal and metacharacter patterns must both be enclosed in slash "/" delimiters, as seen in our example below.)

Since in our example a match is found, the question and answer will be echoed. If you change the word "chicken" in either one of the instances where it is found, there will no longer be a match, and the statements will not be echoed.

Metacharacters are understandably more involved than their literal counterparts, but with a little study, they can be interpreted. Let's begin with an easy subject: character classes.

Character classes specify which characters are acceptable in a pattern. [a-z], [A-B] and [0-9] are all examples of character classes covering a wide range of characters, but you can build your own by enclosing the acceptable characters in square brackets. Examples: [dgefgh] matches d, g, e, f, etc. and [l-p] matches l, m, n, o & p.

Negated character classes specify which characters are not acceptable in a pattern. These classes are in square brackets, but begin with ^. Examples: [^dgefgh] means that d, g, e, f, g & h are not acceptable and [^l-p] means that l, m, n, o & p are not acceptable.

And now let's take a brief look at the basic possibilities that regular expressions allow.

Now that you have all of that memorized, let's jump into our long-awaited example, where we will determine whether or not an email address uses the correct syntax. Consider the following by looking up each symbol on the chart above, and see what you make of it.

/^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,3})$/

As you can see, this regular expression defines a series of rules that express an acceptable pattern to be followed in order to validate the proper syntax of an email address. It can be used in the following manner:

Try playing around with the email address to see what formats are allowing with the regex pattern used, the make up some patterns of your own, the possibilities are endless... well, almost.

8. User-Defined PHP Functions

We have been learning about all kinds of useful functions that come built-in with PHP, but what if you can't find a function to perform a certain task? What if it does not exist? PHP allows you to build your own functions.

Why build a function? Why not just write the code needed to perform the task and move on? If you think you might use a block of code more than once, it will save a lot of time and effort to put it in a function and call the function as many times as necessary later on in the page. You can even store your function(s) in a separate file and include the file on as many pages as needed.

To create a function, the syntax is: function name() { }

You can name a function anything you want, as long as the function name is not already in use as a PHP function. Function names are not case sensitive, and can include letters, numbers and/or underscores, but can only begin with letters or underscores. Let's take a look at a simple function:

So what does this function do? Nothing! Why? Because it has not yet been called. Until a function is called, it will sit around looking bored (or boring), waiting to be of use... Let's learn how to call our poor function and give it something to do:

Simple! Too simple? Probably. All that the function does now that it is called is echo a single statement. Let's try something a bit more complex.

Now, when our new function is defined, we have two parameters, or variables, between the function's parentheses. These variables are used in the function, where they are multiplied by one another and echoed. The variables are not actually assigned their values until the function is called, at which point the values are listed in the function's parentheses, comma-separated in the order of the variables they need to be assigned to. A function can handle as many (or few) comma-separated parameters as you (the programmer) can keep track of, if necessary.

Returning values allows you to store the results of a user-defined function in a variable. Consider the following:

As you can see, instead of echoing the result of $a + $b, we store it in a variable and then return that variable. (A function can only return one thing, so choose carefully!) Then, the function is called as the value of the $addition variable, and is assigned two values to use in the function. At last, the $addition variable (containing the results of the function) is echoed along with a short string of text.

Some people consider functions hard to get a handle on, but it is not really much harder than learning how to ride a bike, and it is a lot easier on the knees!