合并的艺术：`array_merge`与工会运营商``

array_merge() 和 union 操作符 ( ) 的主要区别在于处理键冲突和索引的方式：1. array_merge() 会重新索引数字键并用后续数组的值覆盖重复的字符串键；2. union 操作符 ( ) 则保留左侧数组的值，不重新索引，适用于设置默认值。应根据是否需要覆盖或保留原有值来选择使用哪种方法，二者各有适用场景而非优劣之分。

When working with arrays in PHP, merging them is a common task—whether you're combining configuration settings, processing form data, or aggregating results. PHP provides two primary ways to combine arrays: the array_merge() function and the union operator ( ). While both can be used to merge arrays, they behave very differently under the hood. Understanding these differences is key to using them correctly and avoiding subtle bugs.

Key Differences in Behavior

The main distinction lies in how they handle duplicate keys, especially when dealing with numeric vs. string (associative) keys.

1. array_merge() — Reindexes and Overwrites

• Numeric keys: Reindexes sequentially, starting from 0.
• String keys: If duplicate keys exist, the later array’s value overwrites the earlier one.
• Null handling: Passing null as an argument throws a warning (since it expects arrays).

$array1 = ['a' => 1, 'b' => 2];
$array2 = ['b' => 3, 'c' => 4];
$result = array_merge($array1, $array2);
// Result: ['a' => 1, 'b' => 3, 'c' => 4]

For numeric keys:

$array1 = [10, 20];
$array2 = [30, 40];
$result = array_merge($array1, $array2);
// Result: [0 => 10, 1 => 20, 2 => 30, 3 => 40]

2. Union Operator ( ) — Preserves First Values

• All keys: Uses the values from the left array for any keys that exist in both arrays.
• No reindexing: Maintains original keys, including numeric ones.
• No overwriting: The left side "wins" on key collision.
• Order matters: $a $b is not the same as $b $a.

$array1 = ['a' => 1, 'b' => 2];
$array2 = ['b' => 3, 'c' => 4];
$result = $array1   $array2;
// Result: ['a' => 1, 'b' => 2, 'c' => 4] — 'b' from $array1 preserved

With numeric keys:

$array1 = [0 => 'x', 1 => 'y'];
$array2 = [1 => 'z', 2 => 'w'];
$result = $array1   $array2;
// Result: [0 => 'x', 1 => 'y', 2 => 'w']

Note: The union operator does not reindex or change numeric keys.

When to Use Each

Use array_merge() when:

• You want to combine lists (e.g., appending items to a result set).
• You’re okay with overwriting duplicate keys.
• You need sequential numeric indexing (like flattening lists).
• Merging associative arrays where newer values should replace older ones (e.g., defaults user input).

$defaults = ['color' => 'red', 'size' => 'M'];
$userPrefs = ['color' => 'blue'];
$final = array_merge($defaults, $userPrefs);
// Result: ['color' => 'blue', 'size' => 'M']

Use the Union Operator ( ) when:

• You want to preserve defaults (e.g., fill missing keys from a fallback array).
• You're defining fallbacks or defaults and don’t want later values to override earlier ones.
• Working with configuration arrays where the primary set should dominate.

$defaults = ['host' => 'localhost', 'port' => 3306, 'debug' => false];
$config = ['host' => 'db.example.com']; // user only set host
$final = $config   $defaults;
// Result: ['host' => 'db.example.com', 'port' => 3306, 'debug' => false]

This pattern is common in libraries and frameworks for setting defaults.

Important Gotchas

• array_merge() requires all arguments to be arrays. Passing null or non-array values triggers a warning.

  array_merge($arr, null); // Warning!

• Union operator only works with arrays. Using it with null or non-arrays results in a fatal error in strict contexts.

• Performance: The union operator is generally faster since it doesn’t reindex or deeply process arrays.

• Associative vs. Indexed: The behavior difference is most noticeable with string keys. With indexed arrays, array_merge() is almost always preferred due to reindexing.

Summary

Feature	array_merge()	Union Operator ( )
Duplicate keys	Later overwrites earlier	Left side wins
Numeric key reindexing	Yes	No
Preserves original keys	No (for numeric)	Yes
Best for	Combining lists, overrides	Setting defaults, fallbacks
Null/safety	Warnings on invalid input	Strict type requirements

In practice:

• Use array_merge() to build up data.
• Use to fill in gaps without overriding.

Choosing the right method comes down to intent: are you layering in new data, or protecting existing values? Once you internalize that, the choice becomes clear.

Basically, it's not about which is better—it's about which fits your use case.

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