Variables and types

A variable takes its type from the value

Every variable in PHP starts with $, and there is no declaration to write: assigning is declaring. $age = 36; makes an integer, $name = "Grace"; makes a string, and the variable can hold a different type tomorrow — PHP is dynamically typed, so the type travels with the value, not with the name.

The scalar types are the ones you will use hourly: int for whole numbers, float for numbers with a decimal point, string for text, bool for true and false, and null for "no value at all". Around them sit array and object, which get chapters of their own, and the callable and resource types at the edges.

Look, do not guess

When a value is not what you expected, ask the language instead of squinting at the output. gettype($x) returns the type as a word; var_dump($x) prints the type and the value together, which is the tool you reach for while debugging, because it also shows the difference between 0, "0", false and null.

var_dump is for looking, not for shipping: it prints in a debugging format. echo prints for the reader. Keeping the two apart is a habit worth forming now.

The output of var_dump is worth reading closely: it names the type, then the value, and it puts quotes around a string. int(0), string(1) "0" and bool(false) look similar in a browser but are three different answers, and the debug view is the only place that difference is visible without guessing.

What null means

null is a value in its own right, not an absence of syntax. It says “there is no value here”, which is different from 0, "" and false — each of those is a real value that a program can use. A variable assigned null exists; a variable that was never assigned does not, and reading it produces a warning.

The distinction matters when data arrives from outside. A form field that was left empty is an empty string, not null; a database column with no value may be null; a function that found nothing may return null on purpose. is_null($value) asks the question directly, and $value === null is the comparison that does not convert anything on the way.

Converting on purpose

PHP converts types on its own when a value is used in a place it does not fit — a number inside a string, a string in arithmetic. That convenience is also the source of PHP's oldest jokes, so the honest habit is to convert deliberately: (int) $text, (float) $text, (string) $number, (bool) $value.

A cast to int of a text that does not start with digits gives 0, and a cast of null gives 0 or an empty string — quietly. That is exactly why chapter five treats == with suspicion: automatic conversion is fine when you asked for it, and a trap when you did not.

Types, asked for and converted

<?php
$name = 'Grace';        // string
$age = 36;              // int
$height = 1.72;         // float
$member = true;         // bool
$note = null;           // null

echo gettype($name) . ' ' . gettype($age) . ' ' . gettype($height) . PHP_EOL;

var_dump($age, $note);  // the debugging view: type and value

// The same variable can move to another type.
$age = 'thirty-six';
echo $age . ' is now a ' . gettype($age) . PHP_EOL;

// A deliberate conversion instead of a lucky one.
$count = (int) '12 apples';
echo $count + 1, PHP_EOL;

The type belongs to the value

A variable is a name in the current symbol table, and the value it points at carries the type. $age = 36; puts an integer in the table under age; a later $age = "thirty-six"; replaces the value and the type together. Nothing about the name remembers the old type.

Assignment copies the value. For scalars that is literal, and for arrays PHP waits until one side changes before duplicating the storage. Objects are the exception, and the classes chapter shows why: a variable holds a handle to the object, so two names can point at the same instance. gettype() reports the type, and it calls a float double — the same type under its older name. is_float() is the question that avoids the vocabulary.

Try these next

• Create one value of each scalar type and print gettype() for all five.
• Cast "3.9" to int and to float; print both and explain the difference.
• Build a total from a numeric string and a float, then cast the result to a string for display.