PHP Quick Guide: The Fundamentals

What is PHP?

PHP (recursive acronym for PHP: Hypertext Preprocessor) is a widely-used open source general-purpose scripting language that is especially suited for web development and can be embedded into HTML. – php.net

PHP is mainly focused on server-side scripting, so you can do anything any other CGI program can do, such as collect form data, generate dynamic page content, or send and receive cookies. But PHP can do much more.

Escaping from HTML

Everything outside of a pair of opening and closing tags is ignored by the PHP parser which allows PHP files to have mixed content. This allows PHP to be embedded in HTML documents.

Will render:

Variables

Variables in PHP are represented by a dollar sign followed by the name of the variable. The variable name is case-sensitive.

Variable names follow the same rules as other labels in PHP. A valid variable name starts with a letter or underscore, followed by any number of letters, numbers, or underscores. As a regular expression, it would be expressed thus: ‘[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*

String Functions

These functions all manipulate strings in various ways.

You can find a full list of string functions here. Some more specialized sections can be found in the regular expression and URL handling sections.

Numbers

Integers

An integer is a number that can be written without a fractional component.

Floating Point Numbers

The term floating point is derived from the fact that there is no fixed number of digits before and after the decimal point; that is, the decimal point can float.

Array

An array is a data structure that stores one or more type of values in a single value.

Associative Array (Hash or Dictionary)

An associative array is  an array with strings as index.

Array Functions

Here some examples of array functions, for a full list of functions click here.

NULL & empty

NULL

The special NULL value represents a variable with no value. NULL is the only possible value of type null.

A variable is considered to be null if:

  • it has been assigned the constant NULL.

  • it has not been set to any value yet.

  • it has been unset().

There is only one value of type null, and that is the case-insensitive constant NULL.

<?php
$var = NULL;       
?>

empty

(PHP 4, PHP 5, PHP 7)

Determine whether a variable is considered to be empty. A variable is considered empty if it does not exist or if its value equals FALSE. empty() does not generate a warning if the variable does not exist.

Note: Prior to PHP 5.5, empty() only supports variables; anything else will result in a parse error. In other words, the following will not work: empty(trim($name)). Instead, use trim($name) == false.

Type Juggling and Typecasting

Type Juggling

PHP does not require (or support) explicit type definition in variable declaration; a variable’s type is determined by the context in which the variable is used. That is to say, if a string value is assigned to variable $var, $var becomes a string. If an integer value is then assigned to $var, it becomes an integer.

Typecasting

Converting an expression of a given type into another type is known as type-casting.

Constants

A constant is an identifier (name) for a simple value. As the name suggests, that value cannot change during the execution of the script (except for magic constants, which aren’t actually constants). A constant is case-sensitive by default. By convention, constant identifiers are always uppercase.

The name of a constant follows the same rules as any label in PHP. A valid constant name starts with a letter or underscore, followed by any number of letters, numbers, or underscores. As a regular expression, it would be expressed thusly: [a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*

if , elseif and else Statements

if

(PHP 4, PHP 5, PHP 7)

The if construct is one of the most important features of many languages, PHP included. It allows for conditional execution of code fragments. PHP features an if structure that is similar to that of C:

if (expr)
  statement

The expression is evaluated to its Boolean value. If expression evaluates to TRUE, PHP will execute statement, and if it evaluates to FALSE – it’ll ignore it.

else / elseif

(PHP 4, PHP 5, PHP 7)

elseif, as its name suggests, is a combination of if and else. Like else, it extends an if statement to execute a different statement in case the original if expression evaluates to FALSE. However, unlike else, it will execute that alternative expression only if the elseif conditional expression evaluates to TRUE.

Logical Operators

Example Name Result
$a and $b And TRUE if both $a and $b are TRUE.
$a or $b Or TRUE if either $a or $b is TRUE.
$a xor $b Xor TRUE if either $a or $b is TRUE, but not both.
! $a Not TRUE if $a is not TRUE.
$a && $b And TRUE if both $a and $b are TRUE.
$a || $b Or TRUE if either $a or $b is TRUE.

The reason for the two different variations of “and” and “or” operators is that they operate at different precedences. (See Operator Precedence.)

Comparison Operators

equal: ==

identical: ===

compare: > < >= <= <>

not equal: !=

not identical: !==

Switch

(PHP 4, PHP 5, PHP 7)

The switch statement is similar to a series of IF statements on the same expression. In many occasions, you may want to compare the same variable (or expression) with many different values, and execute a different piece of code depending on which value it equals to. This is exactly what the switch statement is for.

Loops

While

(PHP 4, PHP 5, PHP 7)

while loops are the simplest type of loop in PHP. They behave just like their C counterparts. The basic form of a while statement is:

while (expr)
    statement

The meaning of a while statement is simple. It tells PHP to execute the nested statement(s) repeatedly, as long as the while expression evaluates to TRUE. The value of the expression is checked each time at the beginning of the loop, so even if this value changes during the execution of the nested statement(s), execution will not stop until the end of the iteration (each time PHP runs the statements in the loop is one iteration). Sometimes, if the while expression evaluates to FALSE from the very beginning, the nested statement(s) won’t even be run once.

For

(PHP 4, PHP 5, PHP 7)

for loops are the most complex loops in PHP. They behave like their C counterparts. The syntax of a for loop is:

for (expr1; expr2; expr3)
    statement

The first expression (expr1) is evaluated (executed) once unconditionally at the beginning of the loop.

In the beginning of each iteration, expr2 is evaluated. If it evaluates to TRUE, the loop continues and the nested statement(s) are executed. If it evaluates to FALSE, the execution of the loop ends.

At the end of each iteration, expr3 is evaluated (executed).

Each of the expressions can be empty or contain multiple expressions separated by commas. In expr2, all expressions separated by a comma are evaluated but the result is taken from the last part. expr2 being empty means the loop should be run indefinitely (PHP implicitly considers it as TRUE, like C). This may not be as useless as you might think, since often you’d want to end the loop using a conditional break statement instead of using the for truth expression.

Foreach

(PHP 4, PHP 5, PHP 7)

The foreach construct provides an easy way to iterate over arrays. foreach works only on arrays and objects, and will issue an error when you try to use it on a variable with a different data type or an uninitialized variable. There are two syntaxes:

foreach (array_expression as $value)
    statement
foreach (array_expression as $key => $value)
    statement

The first form loops over the array given by array_expression. On each iteration, the value of the current element is assigned to $value and the internal array pointer is advanced by one (so on the next iteration, you’ll be looking at the next element).

The second form will additionally assign the current element’s key to the $key variable on each iteration.

It is possible to customize object iteration.

Note:

In PHP 5, when foreach first starts executing, the internal array pointer is automatically reset to the first element of the array. This means that you do not need to call reset() before a foreach loop.

As foreach relies on the internal array pointer in PHP 5, changing it within the loop may lead to unexpected behavior.

In PHP 7, foreach does not use the internal array pointer.

In order to be able to directly modify array elements within the loop precede $value with &. In that case the value will be assigned by reference.

Continue & Break

Continue

(PHP 4, PHP 5, PHP 7)

Continue is used within looping structures to skip the rest of the current loop iteration and continue execution at the condition evaluation and then the beginning of the next iteration.

Note: In PHP the switch statement is considered a looping structure for the purposes of continue. continuebehaves like break (when no arguments are passed). If a switch is inside a loop, continue 2 will continue with the next iteration of the outer loop.

continue accepts an optional numeric argument which tells it how many levels of enclosing loops it should skip to the end of. The default value is 1, thus skipping to the end of the current loop.

Break

(PHP 4, PHP 5, PHP 7)

Break ends execution of the current for, foreach, while, do-while or switch structure.

break accepts an optional numeric argument which tells it how many nested enclosing structures are to be broken out of. The default value is 1, only the immediate enclosing structure is broken out of.

Pointers

There’s an internal implementation for “arrays” in PHP “behind the scenes”, written in C. This implementation defines the details of how array data is actually stored in memory, how arrays behave, how they can be accessed etc. Part of this C implementation is an “array pointer”, which simply points to a specific index of the array.

Defining Functions

A function may be defined using syntax such as the following:

<?php
function say_hello_to($name){
    return "Hello, {$name}!";
}
?>

Any valid PHP code may appear inside a function, even other functions and class definitions.

Function names follow the same rules as other labels in PHP. A valid function name starts with a letter or underscore, followed by any number of letters, numbers, or underscores. As a regular expression, it would be expressed thus: [a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*.

Tip

See also the Userland Naming Guide.

Functions need not be defined before they are referenced, except when a function is conditionally defined as shown in the two examples below.

Function Arguments

Information may be passed to functions via the argument list, which is a comma-delimited list of expressions. The arguments are evaluated from left to right.

PHP supports passing arguments by value (the default), passing by reference, and default argument values. Variable-length argument lists are also supported.

Default Argument Values

A function may define C++-style default values.

Returning Values

Values are returned by using the optional return statement. Any type may be returned, including arrays and objects. This causes the function to end its execution immediately and pass control back to the line from which it was called. See return for more information.

Note: If the return is omitted the value NULL will be returned.

For More information about PHP please read the docs here.

Resources

Adrian Prieto

“If you are not willing to learn, no one can help you. If you are determined to learn, no one can stop you.” ― Zig Ziglar

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