🛡️ PHP Strict Types – Enable Safe and Predictable Code

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🧲 Introduction – Why Strict Typing Matters in PHP

PHP is a flexible, dynamically typed language. While this allows for quick development, it can also introduce hidden bugs due to automatic type conversions (also known as type juggling). Since PHP 7, developers can opt into strict typing to enforce exact data types for function parameters and return values—leading to safer, more reliable, and more predictable code.

🎯 In this guide, you’ll learn:

• What strict typing is in PHP
• How to enable it and when to use it
• Differences between strict and coercive mode
• Best practices and examples for modern PHP development

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📘 What Is Strict Typing in PHP?

Strict typing means PHP will not automatically convert one data type into another. Instead, the data passed to a function must exactly match the declared type — otherwise, PHP throws a TypeError.

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🔧 How to Enable Strict Typing

To enable strict typing in a PHP file, add the following declaration at the top:

<?php
declare(strict_types=1);

⚠️ It must be the very first statement in the file (before any code or whitespace).

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🎯 Example – Without Strict Typing (Default Behavior)

<?php
function add(int $a, int $b) {
    return $a + $b;
}

echo add("5", "10"); // ✅ Outputs: 15 (strings are converted to integers)
?>

✅ PHP automatically converts "5" and "10" to integers — this is called coercive mode.

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🚫 Example – With Strict Typing Enabled

<?php
declare(strict_types=1);

function add(int $a, int $b) {
    return $a + $b;
}

echo add("5", "10"); // ❌ Fatal error: must be of the type int
?>

❌ Throws a TypeError because string values are not accepted as integers.

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🔁 Coercive vs Strict Typing in PHP

Mode	Description	Default?
Coercive	PHP converts types automatically	✅ Yes
Strict	Enforces exact type match	❌ No

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📚 Types Affected by Strict Typing

Strict typing applies to:

• Function arguments
• Function return types
• Method parameters and return types

✅ It works with scalar types, including:

• int, float, bool, string
• array, object, callable, iterable
• void, mixed, never, and union/nullable types (PHP 7.1+ and PHP 8+)

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🧪 Example – Function Return Type with Strict Typing

<?php
declare(strict_types=1);

function getStatus(): bool {
    return "true"; // ❌ Fatal error: string returned instead of boolean
}

✅ Use return true; instead of "true".

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✅ Best Practices for Using Strict Typing

Best Practice	Benefit
✅ Always use declare(strict_types=1);	Helps prevent bugs early
✅ Use strict typing in all files	Ensures consistent behavior across your code
✅ Type-hint both parameters and returns	Improves code readability and debugging
✅ Combine with var_dump() or gettype() for debugging	Clarifies type expectations

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🛠️ Real-World Example – Form Validation

declare(strict_types=1);

function validateAge(int $age): bool {
    return $age >= 18;
}

$input = $_POST["age"]; // Comes as a string
echo validateAge((int)$input); // ✅ Explicit casting before passing

✅ Always cast input data to proper types when working with strict mode.

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📌 Summary – Recap & Next Steps

Strict typing is one of the most powerful tools for writing robust PHP code. It prevents unpredictable behavior by enforcing type discipline, especially in large projects, APIs, and team environments.

🔍 Key Takeaways:

• Enable strict typing using declare(strict_types=1) at the top of the file
• Prevents automatic type conversion — ensures exact type matching
• Works with scalar and return types, introduced in PHP 7+
• Helps catch bugs during development, rather than at runtime
• Should be used with manual casting, validation, and type hints

⚙️ Real-World Relevance:
Strict typing is used in frameworks like Laravel, Symfony, and WordPress plugin development, and it’s critical for building APIs, validation systems, and secure business logic.

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❓ Frequently Asked Questions (FAQ)

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❓ Is strict typing required in PHP?
❌ No, it’s optional. PHP uses coercive mode by default unless strict_types=1 is declared.

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❓ Where should declare(strict_types=1) be placed?
✅ It must be the very first line in the file — even before whitespace or comments.

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❓ Does strict typing apply to class properties?
❌ No. It only applies to function/method parameters and return values, not class properties.

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❓ Can I still cast input manually in strict mode?
✅ Yes, and you should! Use (int), (float), etc., to convert user input before passing to typed functions.

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❓ Is there a performance cost to strict typing?
❌ Negligible. The type safety benefits far outweigh any minimal overhead.

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