使用mb_substr() 是解決PHP 中Unicode 字符串截取問題的正確方法，因為substr() 按字節(jié)切割會導(dǎo)致多字節(jié)字符（如emoji 或中文）被截斷成亂碼；而mb_substr() 按字符切割，能正確處理UTF-8 編碼的字符串，確保輸出完整字符，避免數(shù)據(jù)損壞。 1. 始終對包含非ASCII 字符的字符串使用mb_substr()；2. 明確指定'UTF-8' 編碼參數(shù)或提前設(shè)置mb_internal_encoding('UTF-8')；3. 使用mb_strlen() 替代strlen() 以獲取正確的字符數(shù)；4. 檢查mbstring 擴展是否啟用，確保函數(shù)可用；5. 僅在處理純ASCII 或二進制數(shù)據(jù)時才考慮使用substr()。只要涉及用戶輸入、國際化或Web 內(nèi)容，就必須使用mb_substr() 來保證字符串操作的安全性和正確性。

When working with multibyte strings—especially those containing non-ASCII characters like emojis, Chinese, Arabic, or even accented Latin letters—using PHP's standard substr() can lead to garbled output or even broken characters. This is the heart of the Unicode challenge in PHP string manipulation.

The issue arises because substr() operates on bytes , not characters . Since UTF-8 encodes some characters using 2, 3, or even 4 bytes, cutting a string in the middle of a byte sequence results in invalid or corrupted text.

That's where mb_substr() comes in.

Why substr() Fails with Unicode

Consider this string:

 $string = "Hello ?"; // The globe emoji is 4 bytes in UTF-8

If you try:

 echo substr($string, 0, 7); // Trying to get "Hello ?" (7 chars)

You might expect "Hello ?" , but depending on how the bytes align, you could end up with something like "Hello " — a mojibake or "garbage character" — because substr() sliced right through the middle of the 4-byte emoji.

This is not just an edge case — it's a real problem when dealing with user-generated content, internationalization, or APIs handling diverse text.

The Solution: mb_substr()

PHP's Multibyte String Functions , specifically mb_substr() , are designed to handle UTF-8 and other encodings correctly by operating on characters , not bytes.

Basic Syntax

 mb_substr(string $str, int $start, ?int $length = null, ?string $encoding = null)

To safely slice the earlier example:

 $safe = mb_substr($string, 0, 7, 'UTF-8');
echo $safe; // Output: "Hello ?" — intact and correct

Key points:

• The fourth parameter ( 'UTF-8' ) explicitly tells PHP the encoding.
• You can omit it if mb_internal_encoding() is set to UTF-8 (which it should be).
• Always specify the encoding when in doubt — don't rely on defaults.

Best Practices for Safe String Slicing

To avoid Unicode-related bugs, follow these guidelines:

• ? Always use mb_substr() for user-facing or international text
• ? Set internal encoding early:

   mb_internal_encoding('UTF-8');

• ? Use consistent encoding across your app — ensure databases, forms, and outputs are all UTF-8.
• ? Validate input encoding if uncertain:

   if (!mb_check_encoding($string, 'UTF-8')) {
      // Handle or convert
  }

• ? Never assume strlen() or substr() are safe with Unicode

Common Pitfalls to Avoid

• Mixing strlen and mb_substr :
  strlen() returns byte count. Use mb_strlen($string, 'UTF-8') instead.

   $text = "café"; // 5 bytes, 4 characters
  echo strlen($text); // 5
  echo mb_strlen($text); // 4 — correct character count

• Forgetting the encoding parameter :
  If omitted, mb_substr() uses the internal encoding — which might not be UTF-8. Be explicit.

• Assuming mbstring is always enabled :
  It's not part of the PHP core; it's an extension. Check with:

   if (!function_exists('mb_substr')) {
      die('Multibyte extension required.');
  }

  ---

  When You Might Still Use substr()

  There are rare cases where byte-level access is needed:

  • Binary data (eg, file headers)
  • Performance-critical code with ASCII-only strings
  • Working with encoded payloads (eg, base64)

  But for any human-readable text that might include Unicode, stick with mb_substr() .

  ---

  Using mb_substr() correctly isn't just about avoiding weird symbols — it's about building robust, internationalized applications. The Unicode challenge isn't exotic; it's everyday reality in modern web development.

  So whenever you slice a string, ask: Is this safe for ???? If you're not using mb_substr() , the answer is probably no.

  Basically, just use mb_substr() with 'UTF-8' — it's not much extra effort, and it saves a lot of headaches.

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