PHP: 演算子

Q: When exactly should I use == vs. === ?

Default to === . Use == only when you genuinely need automatic type coercion (e.g., comparing numbers from different sources). For strpos() checks, comparisons with false / null , you must use === .

Q: How do I choose between ?? and ?: ?

Use ?? when pulling values from GET/POST (the user might not send the parameter at all). Use ?: when you care whether a variable has "actual content" (empty string counts as no content). Use ?? for array key existence checks too.

Operators are the building blocks of a programming language — you combine variables and values with them to perform calculations and make decisions. PHP's operators are rich, intuitive, and many borrow directly from mathematical symbols.

1. Arithmetic Operators

▶ サンプル: All Arithmetic Operators

PHP

<?php
$a = 10;
$b = 3;

echo $a + $b;   // 13  Addition
echo $a - $b;   // 7   Subtraction
echo $a * $b;   // 30  Multiplication
echo $a / $b;   // 3.333...  Division (note: PHP division always returns a float)
echo $a % $b;   // 1   Modulus (remainder)
echo $a ** $b;  // 1000  Exponentiation (PHP 5.6+, 10 to the power of 3)

// Negation
echo -$a;       // -10  Negation
?>

▶ 試してみよう

2. Assignment Operators

Basic = assignment is straightforward. The real time-savers are the combined assignment operators:

PHP

<?php
$x = 10;

$x += 5;   // $x = $x + 5;   → 15
$x -= 3;   // $x = $x - 3;   → 12
$x *= 2;   // $x = $x * 2;   → 24
$x /= 4;   // $x = $x / 4;   → 6
$x %= 4;   // $x = $x % 4;   → 2
$x .= " yuan"; // $x = $x . " yuan"; → "2 yuan" (string append assignment)
?>

3. Comparison Operators

Used to compare two values; the result is true or false:

PHP

<?php
$a = 5;
$b = "5";

// Loose comparison (== compares value only, auto-converts types)
var_dump($a == $b);    // bool(true)  — 5 equals "5"
var_dump($a != $b);    // bool(false)

// Strict comparison (=== compares both value and type)
var_dump($a === $b);   // bool(false) — 5 doesn't equal "5" (different types)
var_dump($a !== $b);   // bool(true)

// Greater / less than
var_dump($a > 3);      // bool(true)
var_dump($a >= 5);     // bool(true)
var_dump($a < 10);     // bool(true)
var_dump($a <= 5);     // bool(true)

// Spaceship operator (PHP 7+) <=>
// Returns -1 if left is smaller, 0 if equal, 1 if left is larger
echo 1 <=> 2;   // -1
echo 2 <=> 2;   // 0
echo 3 <=> 2;   // 1
// Commonly used in sorting callbacks
?>

(1) `==` vs. `===` Decision Guide

Scenario	Use	Reason
Form input validation	`===`	User input `"0"` can be misjudged by `==`
Comparing database return values	`===`	Databases may return numbers as strings
Comparing with constants	`===`	Precise comparison
Checking `strpos()` return value	`!== false`	0 would be misjudged by `== false`
Simple numeric comparison (both sides definitely numbers)	`==`	It works, but `===` doesn't hurt either

💡 Tip: Unless you have a clear reason to use == (loose comparison), default to === (strict comparison). This prevents countless bugs caused by implicit type conversion.

▶ サンプル: Strict Comparison in Practice

PHP

<?php
// In a search engine, a user might enter "0"
$search = "0";

// ❌ Wrong approach
if ($search == false) {
    echo "You didn't enter a search term"; // This runs! Because "0" == false
}

// ✅ Correct approach
if ($search === "") {
    echo "You didn't enter a search term"; // "0" !== "", won't misjudge
}
?>

▶ 試してみよう

4. Logical Operators

Used to combine multiple conditions:

PHP

<?php
$age = 20;
$hasTicket = true;

// AND (both must be true)
var_dump($age >= 18 && $hasTicket);   // bool(true)
var_dump($age >= 18 and $hasTicket);  // bool(true) — lower precedence than &&

// OR (at least one must be true)
var_dump($age < 18 || $hasTicket);    // bool(true) — has a ticket, so can enter
var_dump($age < 18 or $hasTicket);    // bool(true) — lower precedence than ||

// NOT (negation)
var_dump(!$hasTicket);  // bool(false)
var_dump(!false);       // bool(true)

// XOR (true when the two are different)
var_dump(true xor false);  // bool(true)
var_dump(true xor true);   // bool(false)
?>

⚠️ Warning: && and and behave differently! $a = true && false; results in false (&& has higher precedence than =), while $a = true and false; results in true (= has higher precedence than and). Always use && and ||, not and and or.

5. String Operators

PHP has only two string operators:

Operator	Action	Example	Result
`.`	Concatenate	`"Hello" . " PHP"`	"Hello PHP"
`.=`	Append	`$s = "A"; $s .= "B";`	$s = "AB"

PHP

<?php
$name = "John";
echo "Hello, " . $name . "!";  // Hello, John!

$html = "<div>";
$html .= "<h1>Title</h1>";
$html .= "<p>Content</p>";
$html .= "</div>";
echo $html;
// <div><h1>Title</h1><p>Content</p></div>
?>

6. Ternary Operator (`?:`)

The ternary operator is a shorthand for if...else:

▶ サンプル: Ternary vs. Null Coalescing Compared

PHP

<?php
$age = 20;

// Format: condition ? value_if_true : value_if_false
$status = ($age >= 18) ? "Adult" : "Minor";
echo $status;  // Adult

// The above is equivalent to:
if ($age >= 18) {
    $status = "Adult";
} else {
    $status = "Minor";
}

// PHP 5.3+ shorthand (omit the middle part)
$username = $_GET['name'] ?: "Guest";
// Equivalent to: use $_GET['name'] if it's truthy, otherwise "Guest"
?>

▶ 試してみよう

💡 Tip: The ternary operator is great for one-line decisions. But never nest ternaries — $a ? $b ? $c : $d : $e is completely unreadable.

7. Null Coalescing Operator (`??`)

This is one of the most practical features introduced in PHP 7:

PHP

<?php
// ?? operator: use the left side if it's not null, otherwise use the right side
$username = $_GET['user'] ?? "Anonymous";
// Equivalent to: isset($_GET['user']) ? $_GET['user'] : "Anonymous"

// ??= operator (PHP 7.4+): assign only if the left side is null
$config = null;
$config ??= "default";   // $config is now "default"

$config2 = "existing value";
$config2 ??= "default";  // $config2 is still "existing value" (not null, so no overwrite)

// The difference between ?? and ?: 
$name = "";
echo $name ?: "default";   // "default" (empty string is falsy)
echo $name ?? "default";   // "" (empty string is not null)
?>

Operator	Logic	How `""` is handled
`?:`	Use the right side if the left is falsy	`""` → use right side
`??`	Use the right side if the left is null or undefined	`""` → use left side (`""` is not null)

💡 Tip: When handling user input, ?? is usually safer than ?: because a user might intentionally enter an empty string "", which is a valid input.

8. Operator Precedence

When an expression has multiple operators, PHP uses precedence to decide which to evaluate first:

Precedence	Operators
High	`**` (exponentiation)
	`!`, `~`, `++`, `--`
	`*`, `/`, `%`
	`+`, `-`, `.`
	`<`, `<=`, `>`, `>=`
	`==`, `!=`, `===`, `!==`
	`&&`
	`
	`??`
	`?:` (ternary)
Low	`=`, `+=`, `.=` , etc.

PHP

<?php
// Don't memorize the table — use parentheses to clarify intent
$result = $a + $b * $c;        // Unclear
$result = $a + ($b * $c);      // Clear

// Same for complex conditions
if (($age >= 18 && $hasTicket) || $isVip) {
    echo "Entry allowed";
}
?>

💡 Tip: You don't need to memorize the operator precedence table. When in doubt, add parentheses. Code is written for humans; parentheses make your intent immediately obvious.

❓ よくある質問

Q When exactly should I use == vs. ===?

A Default to ===. Use == only when you genuinely need automatic type coercion (e.g., comparing numbers from different sources). For strpos() checks, comparisons with false/null, you must use ===.

Q How do I choose between ?? and ?:?

A Use ?? when pulling values from GET/POST (the user might not send the parameter at all). Use ?: when you care whether a variable has "actual content" (empty string counts as no content). Use ?? for array key existence checks too.

Q Why does 1 + "10 cats" produce 11 and not 1?

A When PHP converts a string to a number, it reads from the beginning until it hits the first non-numeric character. "10 cats" starts with "10", so it converts to 10. If the string starts with a non-numeric character (e.g., "cats 10"), it converts to 0.

📖 まとめ

Use . to concatenate strings and .= to append (not +)
=== performs strict comparison (value and type) — prefer it by default
<=> spaceship operator returns -1/0/1 — perfect for sorting
&& has higher precedence than and — only use && and ||
?? checks for null/undefined; ?: checks for falsy — prefer ?? for user input
When unsure about precedence, add parentheses; clarity beats brevity

📝 練習問題

Create two variables representing a user's age and membership status, then use logical operators to determine whether they can enter (age ≥ 18 OR is a member).
Use the ?? operator to safely retrieve the URL parameter page with a default of 1; then use the ternary operator to display "Homepage" if page ≤ 1, or "Page N" otherwise.
Write a combined operator exercise: given a product price and discount rate, calculate the discounted price and format the output to two decimal places.

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私たちについて

PHP: 演算子

1. Arithmetic Operators

▶ サンプル: All Arithmetic Operators

2. Assignment Operators

3. Comparison Operators

(1) == vs. === Decision Guide

▶ サンプル: Strict Comparison in Practice

4. Logical Operators

5. String Operators

6. Ternary Operator (?:)

▶ サンプル: Ternary vs. Null Coalescing Compared

7. Null Coalescing Operator (??)

8. Operator Precedence

❓ よくある質問

📖 まとめ

📝 練習問題

(1) `==` vs. `===` Decision Guide

6. Ternary Operator (`?:`)

7. Null Coalescing Operator (`??`)