Conditions in PHP - IF() {...} - branching options

> **Note:** This article may be a bit messy for some beginners as it assumes a basic understanding of PHP. If you're interested in how conditions work, read about conditions in the beginner's course.

Description

Often you need to determine if an equality holds or if a statement is true, this is what conditions are for. PHP uses the following syntax like many other languages (especially C):

if (/* logical statement */) {
    // construct
}

Each logical expression has a value of TRUE (true) or FALSE (false), there are no other options.

Example of comparing whether the variable $x is greater than the variable $y:

$x = 10;
$y = 5;

if ($x > $y) {
    // This part of the script is executed if the condition is true
} else {
    // This part of the script will run if the condition does not apply
}

The condition construct has a mandatory content in round brackets, in which the expression to be tested is given, composed of operators (overview below), multiple expressions can be linked using logical operators (overview below).

In addition, the condition contains two optional statement blocks.

• If the condition holds
• If the condition does not hold

For practical reasons, always include at least the first block of statements when the condition holds, otherwise testing the expression would not make sense.

Use of semicolons and parentheses

In general:

• Round parentheses are used to separate logical expressions (other round brackets can be plunged to achieve more complex expressions).
• The complex bracket is used to delimit a block of commands and functions.
• The middle is not used to indicate a condition (the block of commands is delimited by a compound bracket), but to separate the individual commands within the condition).

The only possible notation with a semicolon (except when using the endif construct):

if ($x > $y);

However, such a condition is meaningless because in both cases the result of the comparison will be discarded and no statement belonging to the condition will be executed.

Alternative notation

Under certain circumstances, the if construct can be used with the omission of the compound brackets. This can only be achieved in the following cases:

• If we execute only one statement in the condition.
• If we use a colon and an endif instead of compound brackets;
• If we use "in-line" notation.

For more detailed information, see the following 3 chapters.

1. Only one command ~ abbreviated syntax

If you are creating a condition in which you want to execute only one construct (statement), you can use either the classic compound bracket notation:

if ($x > 10) { $y = $x; }

Or we can omit the brackets:

if ($x > 10) $y = $x;

However, this behavior only applies to a single command in the immediate vicinity of the condition.

A better example (only the $y = $x construct is executed conditionally, the rest is always executed):

$x = 5;
$y = 3;
$z = 10;
if ($x > $y)
$y = $x;
$x = 3;

2. Colon and endif;

if (/* expression */):
    konstrukt;
    konstrukt;
    konstrukt;
endif;

However, this notation has long been considered obsolete because it reduces in orientation when multiple conditions are immersed in themselves.

Note: I would like to note that this style is also liked by some people, such as Yuh (see his article). God forbid you use this somewhere.

3. Ternary expression ~ single line "in-line" notation

Occasionally it is useful to do a simple in-line comparison with some other action (for example, along with defining a new variable). If we want to execute only one statement, the whole procedure can be reduced to a single line even while keeping it as simple as possible.

$x = 5;
$isBiggerThanTwo = ($x > 2 ? true : false);

// or even shorter:

$isBiggerThanTwo = ($x > 2);

// or without brackets:

$isBiggerThanTwo = $x > 2;

Operator tables

Two types of operators are used within the condition:

• Comparative ~ they compare a specific relationship,
• Logical ~ combine multiple expressions to create complex conditions.

Comparative operators

Example (valid when $x is not 5):

if ($x != 5) { ... }

Logical operators

A more complex example:

$x = 5;
$y = 3;
$z = 8;
if ($x > 0 && !($y != 2 && $z == $x) || $z > $y) { ... }

Omitting logical and comparison operators

Often we can afford to omit either operator (or even both), but we must never forget the rules of proper usage to make the resulting expression work.

In general, when testing an expression without an operator, we test whether its value is TRUE or non-empty (for example, it contains a non-zero number, a non-empty string, ...).

Examples:

$x = 5;
$y = 3;
$z = 8;

if ($x) { ... }         // passes because $x is not empty
if ($x && $y) { ... }   // passes because $x and $y are not empty
if (!$x) { ... }        // fails because TRUE is negated
if (isset($z)) { ... }  // passes because the variable $z exists

But tricky situations can arise, especially when:

• I ask for if ($x) and the variable $x contains zero (0), then the condition is not satisfied.
• Or the variable $x contains the string 0 (the number zero), because it overflows to zero and the expression is therefore not true.
• There is a function in the condition that always returns some non-empty string, because then the condition is always true.
• Or if we are checking input from the user and he returns 'false' as a string, then again the condition is true because the string is non-empty.

I recommend one simple and effective solution to this - ask for the number of characters that are returned. If the string is empty (or the variable does not exist), then zero characters are returned and the condition is not satisfied. Simple example:

$x = '0';
if ($x) { ... }			// condition does not normally apply
if (strlen($x)) { ... }	// condition is valid because $x contains 1 character

Next, we can test for the existence of a variable using the isset() function.

String comparison

Finding out that the strings are identical is easy:

$a = 'Cat';
$b = 'cat';

if ($a === $b) {
    // If the strings are the same
} else {
    // If the strings are different
}

It is important to keep a proper eye on the data types in case the entry might be equivalent to some other one.

For example, the empty string $a = ''; is different from the string NULL: $b = NULL;. We need to make this distinction, for example, because of databases where there is a difference between a value not existing or being empty.

$a = '';
$b = null;

if ($a == $b) {
    // It will be evaluated as TRUE because
    // the data type is converted.
}

if ($a === $b) {
    // Performs much more rigorous validation
    // and it won't pass because it's a different
    // content and a different data type, therefore
    // this code will never run.
}

It is also a good idea to ignore white (invisible) characters such as spaces, tabs and line breaks when comparing strings. This is useful, for example, when entering a password and passing it to a hashing function:

$password = '81dc9bdb52d04dc20036dbd8313ed055'; // 1234
$userPassword = '1234';

if (md5(trim($userPassword)) === $password) {
    // The trim() function automatically deletes spaces.
}

Unknown (non-existent) value?

Sometimes it can happen that the value does not exist (it is neither TRUE nor FALSE), it is mainly a value obtained from the database (for example, we are asking for a column that does not exist), in this case the data type NULL will be returned.

In general, NULL is evaluated as FALSE, i.e. the condition does not apply. However, this behavior is not always convenient, since a non-existent value does not necessarily mean that there is no record.

Example from practice: We have a user profile and we query the user's web page. Not all users need to have a web page, so in this case NULL is returned, but the user still exists. So in this case, we should rather use the isset() function to test for the (non-)existence of the variable and not make a conclusion based on a specific value.

Jan Barášek   Více o autorovi

Autor článku pracuje jako seniorní vývojář a software architekt v Praze. Navrhuje a spravuje velké webové aplikace, které znáte a používáte. Od roku 2009 nabral bohaté zkušenosti, které tímto webem předává dál.

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