The methods to merge two PHP array variables are: 1. Use the array_merge() function to merge the index or associative arrays, the numeric index will be renumbered, and the string key will overwrite the previous value; 2. Use operators to retain the key value of the first array, and the same key in the second array will not overwrite; 3. Use array_replace() or array_replace_recursive() for top-level or recursive replacement; 4. Append elements one by one to the end of another array through a loop.

Merging two PHP array variables is actually quite common, especially when processing data or obtaining information from multiple sources. The key is how you want to combine - whether to keep the index unchanged, or reorder, or overwrite duplicate key values? The following are several practical methods that are suitable for different scenarios.

1. Use array_merge() function (most commonly used)

This is the most intuitive way to "connect" two arrays. If it is an index array (numeric subscript), it will automatically re-index; if it is an associative array (string key name), the subsequent array will overwrite the previous key of the same name.

 $array1 = ['a', 'b'];
$array2 = ['c', 'd'];

$result = array_merge($array1, $array2);
// Output: ['a', 'b', 'c', 'd']

If it is an associative array:

 $array1 = ['name' => 'Alice', 'age' => 25];
$array2 = ['age' => 30, 'city' => 'Beijing'];

$result = array_merge($array1, $array2);
// Output: ['name' => 'Alice', 'age' => 30, 'city' => 'Beijing']

Note:

• If it is a numeric index, the key will be renumbered.
• If it is a string key, the following value will overwrite the previous one.

2. Use Operator (preserves the key value of the first array)

If you don't want the following array to overwrite the previous key value, you can use operator. It preserves the key values ??in the first array, and will not be replaced even if the second array has the same key.

 $array1 = ['name' => 'Alice', 'age' => 25];
$array2 = ['age' => 30, 'city' => 'Beijing'];

$result = $array1 $array2;
// Output: ['name' => 'Alice', 'age' => 25, 'city' => 'Beijing']

This method is suitable for situations where you want to "supplement" the data rather than overwrite it.

3. Use array_replace() or array_replace_recursive() (deep replacement)

If you want to control the replacement behavior more accurately, such as replacing only certain levels of data, you can consider these two functions:

• array_replace() : will only replace the same keys on the top layer.
• array_replace_recursive() : Will recursively replace nested arrays.

example:

 $array1 = ['user' => ['name' => 'Alice', 'age' => 25]];
$array2 = ['user' => ['age' => 30]];

$result1 = array_replace($array1, $array2);
// Output: ['user' => ['age' => 30]]

$result2 = array_replace_recursive($array1, $array2);
// Output: ['user' => ['name' => 'Alice', 'age' => 30]]

4. Add elements directly with [] (suitable for appending single or multiple elements)

Sometimes you don't need to merge the entire array, just want to add the contents of one array to the end of another array, you can also use [] directly:

 $array1 = ['a', 'b'];
$array2 = ['c', 'd'];

foreach ($array2 as $item) {
    $array1[] = $item;
}
// Result: ['a', 'b', 'c', 'd']

Although not as concise as array_merge() , it is occasionally used when splicing arrays in loops.

Basically these are the methods. The scenarios that different methods are applicable are slightly different. The key is to choose the appropriate method according to your needs. For example, whether the same key needs to be overwritten, whether recursive merge is needed, etc.

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