PHP的類型轉(zhuǎn)換靈活但需謹(jǐn)慎，易引發(fā)隱性bug；1. 字符串轉(zhuǎn)數(shù)字時(shí)提取開(kāi)頭數(shù)值，無(wú)數(shù)字則為0；2. 浮點(diǎn)轉(zhuǎn)整數(shù)向零截?cái)?，不四捨五入?. 僅0、0.0、""、"0"、null和空數(shù)組為false，其餘如"false"也為true；4. 數(shù)字轉(zhuǎn)字符串可能因浮點(diǎn)精度失真；5. 空數(shù)組轉(zhuǎn)布爾為false，非空為true；6. 數(shù)組轉(zhuǎn)字符串恆為"Array"，不輸出內(nèi)容；7. 對(duì)象轉(zhuǎn)數(shù)組保留公有屬性，私有受保護(hù)屬性被修飾；8. 數(shù)組轉(zhuǎn)對(duì)像生成stdClass實(shí)例；9. 資源無(wú)法有效轉(zhuǎn)換，結(jié)果通常為0或""；10. 對(duì)象轉(zhuǎn)字符串需定義__toString()方法；應(yīng)驗(yàn)證輸入、使用filter_var()、is_()函數(shù)檢查類型、避免隱式轉(zhuǎn)換，確保類型安全。

PHP's type casting system is both flexible and forgiving — a double-edged sword that empowers rapid development but can also introduce subtle bugs if misunderstood. At the heart of this system are scalar and compound types, and how PHP behaves when converting between them. Let's break down the nuances of casting in PHP, focusing on practical implications and common pitfalls.

Understanding Scalar Type Casting

Scalar types in PHP include int , float , string , and bool . These are the simplest data types, and casting among them follows predictable but sometimes surprising rules.

Key Casting Behaviors

• String to Number :
  When casting a string to an integer or float, PHP extracts the leading numeric portion:

  

   (int)"123abc" // 123
  (float)"4.5xyz" // 4.5
  (int)"abc123" // 0 (no leading digits)

  This can lead to silent data loss if input isn't validated.

• Float to Int :
  PHP truncates toward zero, not rounds:

  

   (int)3.9 // 3
  (int)-3.9 // -3

  Use round() explicitly if rounding is intended.

• Booleans :
  Only 0 , 0.0 , "" , "0" , null , and empty arrays cast to false . Everything else — including "0.0" , "false" , and " " — is true :

   (bool)"false" // true — a common gotcha

• Numbers to String :
  Straightforward, but be cautious with precision:

   (string)0.1 0.2 // "0.30000000000000004" when converted, due to float imprecision

Compound Types: Arrays, Objects, and Resources

Compound types ( array , object , resource ) behave very differently during casting, and some conversions are lossy or context-dependent.

Array Casting Rules

• Array to Boolean :
  Empty array → false , otherwise true .

   (bool)[] // false
  (bool)[0] // true

• Array to String :
  Always results in the string "Array" , never the contents:

   echo (string)[1,2,3]; // Prints: Array

  This often causes confusion in concatenation:

   "Data: " . [1,2,3] // "Data: Array"

• Object to Array :
  Converts object properties to associative array keys:

   (array) new DateTime() // ['date' => ..., 'timezone' => ...]

  Public properties become keys; private/protected ones are mangled.

• Array to Object :
  Creates a generic stdClass with keys as property names:

   (object)['name' => 'John'] // ->name === 'John'

Objects and Resources

• Resource to Anything :
  Resources (like file handles) cannot be meaningfully cast. Attempting to cast to string or int usually results in 0 or "" , and may trigger warnings.

• Object to String :
  Only works if the class defines a __toString() method:

   (string)new DateTime(); // Works — has __toString()
  (string)new stdClass(); // Fatal error without __toString()

Practical Tips for Safer Casting

To avoid surprises, follow these guidelines:

• ? Validate input before casting — especially when dealing with user data.
• ? Use strict comparison ( === ) after casting to ensure expected types.
• ? Prefer filter_var() over raw casting for sanitization:

   filter_var($input, FILTER_VALIDATE_INT) // Returns int or false

• ? * Use `is_ ()` functions** to check types before casting:

   if (is_numeric($value)) { ... }

• ? Avoid relying on implicit casting in conditionals or arithmetic.

Final Notes

PHP's casting is convenient but demands awareness. Scalar casts often “do something,” even when that something isn't what you expect. Compound types lose data or fail silently in unexpected ways. The key is to treat casting not as a magic fix, but as a deliberate operation — best paired with validation and type checking.

Basically: cast with care, verify with intent.

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