Conditionals and Logic in PHP

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PHP else statement

A PHP else statement can follow an if block. If the condition of the if does not evaluate to TRUE, the code block following else will be executed.

$condition = FALSE;
if ($condition) {
  // This code block will not execute
} else {
  // This code block will execute
}

PHP comparison operators

PHP comparison operators are used to compare two values and return TRUE or FALSE depending on the validity of the comparison. Comparison operators include:

identical (===)
not identical (!==)
greater than (>)
less than (<)
greater than or equal (>=)
less than or equal (<=)

// Comparison operators
1 > 3; // FALSE
3 > 1; // TRUE
250 >= 250; // TRUE
1 === 1; // TRUE
1 === 2; // FALSE
1 === "1"; // FALSE

PHP If Statements

PHP if statements evaluate a boolean value or expression and execute the provided code block if the expression evaluates to TRUE.

if (TRUE){
  echo "TRUE is always true"; 
}

$condition1 = TRUE;
if ($condition1) {
  // This code block will execute
}

$condition2 = FALSE;
if ($condition2) {
  // This code block will not execute
}

PHP switch statement

PHP switch statements provide a clear syntax for a series of comparisons in which a value or expression is compared to many possible matches and code blocks are executed based on the matching case.

In PHP, once a matched case is encountered, the code blocks of all subsequent cases (regardless of match) will be executed until a return, break, or the end of the statement is reached. This is known as fall through.

switch ($letter_grade){
  case "A":
    echo "Terrific";
    break;
  case "B":
    echo "Good";
    break;
  case "C":
    echo "Fair";
    break;
  case "D":
    echo "Needs Improvement";
    break;
  case "F":
    echo "See me!";
    break;
  default:
    echo "Invalid grade";
}

PHP readline()

The PHP built-in readline() function takes a string with which to prompt the user. It waits for the user to enter text into the terminal and returns that value as a string.

echo "\nWhat's your name?\n";
$name = readline(">> "); // receives user input

PHP elseif statements

PHP elseif statements must be paired with an if statement, but many elseifs can be chained from a single if.

elseifs provide an additional condition to check (and corresponding code to execute) if the conditional statements of the if block and any preceding elseifs are not met.

$fav_fruit = "orange";

if ($fav_fruit === "banana"){
  echo "Enjoy the banana!";
} elseif ($fav_fruit === "apple"){
  echo "Enjoy the apple!";
} elseif ($fav_fruit === "orange"){
  echo "Enjoy the orange!";
} else {
  echo "Enjoy the fruit!";
}
// Prints: Enjoy the orange!

PHP Truthy and Falsy

PHP values within a condition will always be evaluated to TRUE or FALSE. Values that will evaluate to TRUE are known as truthy and values that evaluate to FALSE are known as falsy.

Falsy values include:

false
0
empty strings
null
undefined
NaN.

All other values are truthy.

if ("What's going on?"){   // evaluates to TRUE
  echo "Let us explain…";
} 
// Prints: Let us explain…

PHP Boolean Values

PHP Boolean values are either TRUE or FALSE, which are the only members of the boolean type

// booleans
$is_true = TRUE;
$is_false = FALSE;

echo gettype($is_true); 
// Prints: boolean
echo gettype($is_false); 
// Prints: boolean

PHP ternary operator

In PHP, the ternary operator allows for a compact syntax in the case of binary (if/else) decisions. It evaluates a single condition and executes one expression and returns its value if the condition is met and the second expression otherwise.

The syntax for the ternary operator looks like the following:

condition ? expression1 : expression2

// Without ternary
$isClicked = FALSE;
if ($isClicked) {
  $link_color = "purple";
} else {
  $link_color = "blue";
}
// $link_color = "blue";


// With ternary
$isClicked = FALSE;
$link_color = $isClicked ? "purple" : "blue";
//  $link_color = "blue";

PHP Nested Conditionals

In PHP, nested conditional statements deepen the complexity of our programs’ decision-making capabilities. They allow us to create programs where each decision made sends our program on a different route where it might encounter additional decisions.

$num = 5;
  
// nested conditional
if ($num > 0){
    echo 'The number is positive. <br>';
	  if ($num % 2 === 0){
		    echo 'The number is even.';
	  }
} else {
	  echo 'The number is negative.';
}

PHP Logical Operators

In PHP, expressions that use logical operators evaluate to boolean values. Logical operators include:

or (||)
and (&&)
exclusive or (xor)
not (!)

// Examples of Logical Operators:

TRUE || TRUE;   // Evaluates to: TRUE
FALSE || TRUE;  // Evaluates to: TRUE
TRUE && TRUE;    // Evaluates to: TRUE
FALSE && TRUE;   // Evaluates to: FALSE
!TRUE;    // Evaluates to: FALSE
!FALSE;   // Evaluates to: TRUE
TRUE xor TRUE;   // Evaluates to: FALSE
FALSE xor TRUE;  // Evaluates to: TRUE

PHP && Operator

The logical operator && returns:

TRUE only if both of its operands evaluate to true.
FALSE if either or both of its operands evaluate to false.

TRUE && TRUE;    // Evaluates to: TRUE
FALSE && TRUE;   // Evaluates to: FALSE
TRUE && FALSE;   // Evaluates to: FALSE
FALSE && FALSE;  // Evaluates to: FALSE

$passingGrades = TRUE;
$extracurriculars = TRUE;
if ($passingGrades && $extracurriculars){
  echo "You meet the graduation requirements.";
}
// Prints: You meet the graduation requirements.

PHP ! (not) Operator

In PHP, the not operator (!) is used to invert a Boolean value or expression.

!TRUE;    // Evaluates to: FALSE
!FALSE;   // Evaluates to: TRUE

PHP Operator Precedence

Each operator in PHP holds a different operator precedence.

We can avoid operator precedence confusion by using parentheses for force the evaluation we want.

TRUE || TRUE && FALSE // Evaluates to: TRUE
(TRUE || TRUE) && FALSE // Evaluates to: FALSE

PHP Xor Operator

In PHP, the logical operator xor stands for exclusive or.

It takes two different boolean values or expressions as its operands and returns a single boolean value.

xor evaluates to TRUE only if either its left operand or its right operand evaluate to TRUE, but not both.

TRUE xor TRUE;   // Evaluates to: FALSE
FALSE xor TRUE;  // Evaluates to: TRUE
TRUE xor FALSE;  // Evaluates to: TRUE
FALSE xor FALSE; // Evaluates to: FALSE

Logical Operators - Alternate Syntax

PHP provides an alternate syntax for the || operator — the or operator.

It also provides an alternate syntax for && operator — the and operator.

These operators have the advantage of making our code more human readable.

// The or Operator:
TRUE or TRUE;   // Evaluates to: TRUE
FALSE or TRUE;  // Evaluates to: TRUE
TRUE or FALSE;  // Evaluates to: TRUE
FALSE or FALSE; // Evaluates to: FALSE

// The and Operator:
TRUE and TRUE;   // Evaluates to: TRUE
FALSE and TRUE;  // Evaluates to: FALSE
TRUE and FALSE;  // Evaluates to: FALSE
FALSE and FALSE; // Evaluates to: FALSE

Multi-File Programs: include

A way to improve our code and separate concerns is with modularity, separating a program into distinct, manageable chunks where each provides a piece of the overall functionality. Instead of having an entire program located in a single file, code is organized into separate files.

In PHP, files can be included in another file with the keyword include. An include statement is followed by a string with a path to the file to be included. The code from the file will be executed.

// one.php
echo "How are";

// two.php
echo " you?";

// index.php
echo "Hello! ";
include "one.php";
include "two.php";
// Prints: Hello! How are you?