Type Juggling

PHP converts types automatically when an operation needs it. This is called type juggling. It is convenient, and it is also a classic source of bugs.

Numbers inside strings

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<?php
var_dump("5" + "5");
var_dump("5" . "5");
var_dump("2.5" * 2);

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int(10)
string(2) "55"
float(5)

The + operator wants numbers, so PHP converts the strings. The . operator wants strings, so it joins them.

Strings that only start like a number

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<?php
var_dump("5 apples" + 1);

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Warning: A non-numeric value encountered in /path/to/index.php on line 2
int(6)

Strings that are not numeric at all

Since PHP 8, arithmetic on a string that is not a number throws an error.

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<?php
try {
    echo "abc" + 1;
} catch (TypeError $e) {
    echo $e->getMessage();
}

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Unsupported operand types: string + int

Loose comparison

The == operator also juggles types, and PHP 8 made these rules saner.

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<?php
var_dump(0 == "a");        // false in PHP 8, was true in PHP 7
var_dump("1" == "01");     // both numeric strings, so compared as numbers
var_dump(100 == "1e2");    // "1e2" is the number 100
var_dump(null == false);

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bool(false)
bool(true)
bool(true)
bool(true)

Why this is dangerous

A loose comparison can let a wrong value through.

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<?php
$role = "0";

if ($role == false) {
    echo "Treated as no role";
}

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Treated as no role

The safe habit

• Use === and !== instead of == and !=. See Equality vs Identity.
• Turn on strict typing in your files.
• Convert on purpose with a cast, so the next person sees what you meant.