PHP Data Types

PHP Data Types: Scalar, Compound, and Special Values

Every value stored in a PHP variable belongs to a specific data type that dictates how PHP stores it and which operations are valid. PHP groups its types into four scalar types, compound types like arrays and objects, and special types like null and resource.

Core PHP Type Categories

Knowing what type a variable holds prevents subtle comparison and math bugs.

Use var_dump() or get_debug_type() during debugging to inspect both the exact type and value of any variable.

<?php
declare(strict_types=1);

$title = "PHP Guide";
$lessonNumber = 7;
$rating = 4.9;
$published = true;
$tags = ["php", "backend", "web"];
$deletedAt = null;

var_dump($title, $lessonNumber, $rating, $published, $tags, $deletedAt);
echo "Detected type: " . get_debug_type($tags) . PHP_EOL;

Type Juggling vs Strict Types

By default, PHP automatically converts compatible types when needed—for instance, turning "10" into 10 during addition. Adding declare(strict_types=1); at the top of your file stops silent type conversions on function calls.

<?php
declare(strict_types=1);

function addTax(float $price, float $rate): float
{
    return $price * (1 + $rate);
}

echo addTax(100.0, 0.15) . PHP_EOL;

Common Mistakes to Avoid

• Confusing empty() with null checks: empty(0) and empty("0") return true. Use $value === null or is_null() when checking specifically for null.
• Comparing floats with === without rounding: Binary floating-point math can produce 0.30000000000000004. Store money as integer cents or use round().
• Using gettype() instead of get_debug_type(): gettype() returns legacy names like "double" instead of "float"; get_debug_type() returns modern PHP 8 type names.

Frequently Asked Questions

What is the difference between NULL and an empty string ""?

null represents the absence of any value, whereas "" is a valid string of length zero.

Can a PHP variable change its type after assignment?

Yes. PHP is dynamically typed, so assigning 42 to $x and later "forty-two" to $x is valid, unless constrained by a typed class property.

How do I safely check if a value is an integer string from a form?

Use filter_var($input, FILTER_VALIDATE_INT) !== false rather than is_int(), because HTML form inputs always arrive as strings.