PHP: Variables and Data Types

Q: Why does echo false show nothing?

Because echo false is equivalent to echo "" (empty string). To inspect a boolean value, use var_dump($var) or echo $var ? 'true' : 'false' .

Q: When should I use isset() vs. empty() ?

isset() checks "does the variable exist and is it not null?" — use it to check if a form was submitted. empty() checks "is the variable empty/zero/false?" — use it to validate whether input is blank. For checking numeric input, use isset() + is_numeric() — never empty() .

Q: PHP is loosely typed. Does that make type bugs common?

Develop two habits: (1) Use === instead of == for comparisons (strict comparison doesn't auto-convert types); (2) Add type declarations to function parameters (PHP 7+ supports this). We'll cover these in detail later.

Variables are like labeled boxes — you put data inside a box, attach a label, and later use the label to find it. In PHP, every variable label must start with $ — that's PHP's most distinctive feature.

1. Declaring and Naming Variables

Declaring a variable in PHP is simple: $ + variable name = value. You don't need to declare a type first, unlike C or Java.

PHP

<?php
$name = "John";       // String
$age = 25;            // Integer
$height = 1.75;       // Float
$isStudent = true;    // Boolean
?>

(1) Naming Rules

Rule	✅ Correct	❌ Wrong
Must start with `$`	`$name`	`name` (no `$`)
Letter or underscore after `$`	`$name`, `$_name`	`$1name`, `$-name`
Followed by letters, digits, underscores	`$user_id`, `$user2`	`$user-id`, `$user@id`
Case-sensitive	`$name` ≠ `$Name`
Use camelCase	`$firstName`, `$userAge`

💡 Tip: PHP variable names starting with $ are inherited from Perl. This makes PHP variables instantly recognizable when mixed with HTML.

2. PHP's Eight Data Types

PHP has 8 primitive types, grouped into three categories:

Category	Type	Example	Description
Scalar	`string`	`"Hello"`	A sequence of characters
	`int`	`42`	Whole numbers
	`float`	`3.14`	Floating-point numbers (also called `double`)
	`bool`	`true` / `false`	Boolean values
Compound	`array`	`[1, 2, 3]`	Ordered map of values
	`object`	`new User()`	Instances of classes
	`callable`	A callback function	Callable type
	`iterable`	Traversable	Iterable type
Special	`null`	`null`	Represents no value
	`resource`	File handle / DB connection	Reference to an external resource

▶ Example: Inspecting Variable Types with var_dump

Output:

TEXT 📖 Display only

string(4) "John"
int(25)
float(19.99)
bool(true)
NULL

PHP

<?php
$name = "John";
$age = 25;
$price = 19.99;
$isValid = true;
$nothing = null;

var_dump($name);     // string(4) "John"
var_dump($age);      // int(25)
var_dump($price);    // float(19.99)
var_dump($isValid);  // bool(true)
var_dump($nothing);  // NULL
?>

3. Strings — Basic Usage

PHP

<?php
$s1 = 'PHP Tutorial';       // Single quotes: literal string, no variable parsing
$s2 = "PHP Tutorial";       // Double quotes: parses variables and escape sequences

$name = "John";
echo 'Hello $name';         // Hello $name  — single quotes don't parse variables
echo "Hello $name";         // Hello John   — double quotes parse variables
?>

Output:

TEXT 📖 Display only

Output displayed

Syntax	Parses variables?	Parses escape sequences?	Best for
`'Single quotes'`	No	Only `\\` and `\'`	Plain text
`"Double quotes"`	Yes	Parses `\n` `\t` etc.	Embedding variables

We'll dive deeper into strings in the next lesson.

▶ Example: Single vs. Double Quotes Compared

PHP

<?php
$name = "John";

// Single quotes: output exactly as written
echo 'Hello, $name!\nNice weather today.';
// Output: Hello, $name!\nNice weather today.

// Double quotes: parse variables and escape sequences
echo "Hello, $name!\nNice weather today.";
// Output: Hello, John! (newline) Nice weather today.
?>

▶ Try it Yourself

4. Integers (int) and Floats (float)

PHP

<?php
$decimal = 42;         // Decimal
$hex = 0x2A;           // Hexadecimal (=42)
$octal = 052;          // Octal (=42)
$binary = 0b101010;    // Binary (=42)

$float1 = 3.14;        // Float
$float2 = 1.2e3;       // Scientific notation = 1200
$float3 = 7E-10;       // Scientific notation = 0.0000000007
?>

Output:

TEXT 📖 Display only

Output displayed

⚠️ Warning: Floating-point numbers can't precisely represent decimal fractions. 0.1 + 0.2 in PHP does not equal 0.3 — it's something like 0.30000000000000004. When working with money, store amounts in cents as integers or use the bcmath extension.

5. Booleans (bool)

Booleans have only two values: true and false. They're case-insensitive, but the convention is lowercase.

PHP

<?php
$isLogin = true;
$isAdmin = false;

if ($isLogin) {
    echo "Welcome back!";
}
?>

💡 Tip: echo true outputs 1; echo false outputs an empty string (nothing shows). Use var_dump() to inspect boolean values.

6. Type-Checking Functions

PHP

<?php
$name = "John";
$age = 25;

var_dump(is_string($name));   // bool(true)
var_dump(is_int($age));       // bool(true)
var_dump(is_float($age));     // bool(false)
var_dump(is_bool($age));      // bool(false)
var_dump(is_array($name));    // bool(false)

// gettype() returns the type name (for debugging, not logic)
echo gettype($name);  // string
?>

Function	Checks For
`is_string()`	String
`is_int()`	Integer
`is_float()`	Float
`is_bool()`	Boolean
`is_array()`	Array
`is_null()`	NULL
`is_numeric()`	Number or numeric string

7. Type Conversion

PHP is a loosely typed language and automatically converts types. You can also convert explicitly:

PHP

<?php
// Automatic conversion
$result = "10" + 5;         // 15 ("10" converted to number)
$text   = "10" . 5;         // "105" (5 converted to string)

// Explicit conversion
$intVal   = (int) "123";    // 123
$floatVal = (float) "3.14"; // 3.14
$strVal   = (string) 123;   // "123"
$boolVal  = (bool) 1;       // true
?>

▶ Example: Type Conversion in Practice

Output:

TEXT 📖 Display only

value
Price: " . value . " yuan

PHP

<?php
// String to number
$a = (int) "42";
echo $a + 8;  // 50

// Number to string
$b = (string) 100;
echo "Price: " . $b . " yuan";  // Price: 100 yuan

// Boolean conversion rules: these values are treated as false
echo (int)((bool)0);     // 0
echo (int)((bool)"");    // 0
echo (int)((bool)[]);    // 0
echo (int)((bool)null);  // 0
// All other values are true
?>

Output:

TEXT 📖 Display only

Output displayed

⚠️ Warning: "0", "", 0, [], and null are all false in a boolean context. If you can't remember, just test it with var_dump((bool)$x).

8. isset() and empty()

These are three of the most frequently used variable-handling functions in PHP:

PHP

<?php
$var1 = "Hello";
$var2 = null;

// isset(): variable is set and not null → true
var_dump(isset($var1));       // bool(true)
var_dump(isset($var2));       // bool(false)
var_dump(isset($notExist));   // bool(false) — doesn't exist

// empty(): variable is empty → true ("", 0, "0", null, false, [] are all "empty")
var_dump(empty($var1));       // bool(false) — "Hello" is not empty
var_dump(empty($var2));       // bool(true)  — null is empty
var_dump(empty(0));           // bool(true)  — 0 is considered empty!
var_dump(empty($notExist));   // bool(true)  — not existing is also considered empty
?>

▶ Example: isset vs. empty Side by Side

Output:

TEXT 📖 Display only

 | empty: " . (empty(42) ? 'true' : 'false') . "<br>

PHP

<?php
function testVar(string $label, mixed $value): void {
    echo "$label — isset: " . (isset($value) ? 'true' : 'false');
    echo " | empty: " . (empty($value) ? 'true' : 'false') . "<br>";
}

testVar('"Hello"', "Hello");
testVar('""', "");
testVar('0', 0);
testVar('null', null);
// Observe the isset and empty results for each value
?>

Output:

TEXT 📖 Display only

Output displayed

🔥 Common Mistake: empty(0) returns true! The number 0 is considered "empty." When validating whether a number was entered, use isset() combined with is_numeric() — don't use empty().

❓ FAQ

Q Why does echo false show nothing?

A Because echo false is equivalent to echo "" (empty string). To inspect a boolean value, use var_dump($var) or echo $var ? 'true' : 'false'.

Q When should I use isset() vs. empty()?

A isset() checks "does the variable exist and is it not null?" — use it to check if a form was submitted. empty() checks "is the variable empty/zero/false?" — use it to validate whether input is blank. For checking numeric input, use isset() + is_numeric() — never empty().

Q PHP is loosely typed. Does that make type bugs common?

A Develop two habits: (1) Use === instead of == for comparisons (strict comparison doesn't auto-convert types); (2) Add type declarations to function parameters (PHP 7+ supports this). We'll cover these in detail later.

📖 Summary

Variables start with $, use camelCase naming, and are case-sensitive
PHP has 8 types: 4 scalar + 4 compound/special
var_dump() shows a variable's value and type — your go-to debugging tool
Double quotes parse variables; single quotes don't
(int), (string), etc. perform explicit type casting
isset() checks existence and non-null; empty() checks emptiness (0 is also considered empty)

📝 Exercises

Create 3 variables of different types (string, integer, boolean) and use var_dump() and gettype() to inspect each one.
Write code to test how isset() and empty() behave on these values: "Hello", "", 0, null, and an undefined variable. Summarize the differences.
Write a type-conversion exercise: take a user-input price string like "$99.9" and convert it into a computable float. Try (float) — does it work directly? If not, think about how to handle it.

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PHP: Variables and Data Types

1. Declaring and Naming Variables

(1) Naming Rules

2. PHP's Eight Data Types

▶ Example: Inspecting Variable Types with var_dump

3. Strings — Basic Usage

▶ Example: Single vs. Double Quotes Compared

4. Integers (int) and Floats (float)

5. Booleans (bool)

6. Type-Checking Functions

7. Type Conversion

▶ Example: Type Conversion in Practice

8. isset() and empty()

▶ Example: isset vs. empty Side by Side

❓ FAQ

📖 Summary

📝 Exercises

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