In PHP, the key difference between array_merge() and the operator lies in how they handle duplicate keys and reindexing. 1) With numeric keys, array_merge() reindexes starting from 0, while preserves original keys and ignores duplicates from the second array. 2) For string keys, array_merge() overwrites values from earlier arrays, whereas keeps the first occurrence. 3) Neither performs recursive merging by default, but array_merge_recursive() can be used for that purpose. 4) Use array_merge() to allow overwriting and clean reindexing, and to preserve first values or provide defaults.

In PHP, combining arrays might seem straightforward, but there are some important differences between using array_merge() and the union operator when working with arrays. They don’t always behave the same way — especially when dealing with numeric or string keys.

How they handle numeric keys

When you merge arrays with numeric keys, the behavior of array_merge() and starts to differ significantly.

• array_merge() will reindex numeric keys starting from 0.
• The operator will preserve the original keys, and if there are duplicate keys, it keeps the first occurrence only.

Example:

$a = [10, 20];
$b = [30, 40];

// Using array_merge
print_r(array_merge($a, $b));
// Output: [0 => 10, 1 => 20, 2 => 30, 3 => 40]

// Using  
print_r($a   $b);
// Output: [0 => 10, 1 => 20]

So in this case:

• array_merge adds all values and reindexes them.
• ignores values from the second array if keys already exist in the first one.

Behavior with string keys (associative arrays)

With associative arrays (where keys are strings), both methods replace duplicate keys, but again in different ways.

• array_merge() will overwrite values with the same key from later arrays.
• will keep the value from the first array and ignore duplicates in the second one.

Example:

$a = ['name' => 'Alice', 'age' => 25];
$b = ['age' => 30, 'city' => 'Paris'];

print_r(array_merge($a, $b));
// Output: ['name' => 'Alice', 'age' => 30, 'city' => 'Paris']

print_r($a   $b);
// Output: ['name' => 'Alice', 'age' => 25, 'city' => 'Paris']

So:

• If you want later values to take precedence, use array_merge().
• If you want to preserve earlier values and treat the second array as a fallback, use .

Handling of nested arrays

Neither array_merge() nor does a recursive merge by default. But how they handle nested arrays still matters depending on your use case.

If two arrays have the same key pointing to another array, both approaches will just replace or skip the entire value — not merge the sub-arrays recursively.

If that’s what you need, consider using array_merge_recursive() or a custom recursive function instead.

Practical advice for choosing between them

Here’s a quick guide on when to use each:

• Use array_merge() when:

  • You want to combine arrays and allow later values to overwrite earlier ones.
  • You're working with numerically indexed arrays and want clean reindexing.

• Use when:

  • You want to provide defaults — keeping the first value if a key exists in both.
  • You're merging associative arrays and want to avoid overwriting existing values.

Also note:

• works only with arrays; trying to use it with something else triggers an error.
• array_merge() can accept multiple arrays and even non-array arguments if you’re not strict.

So basically, the main difference comes down to how they deal with duplicate keys and whether they reindex numeric keys. Depending on whether you want to preserve or overwrite data, pick the right tool for the job.

The above is the detailed content of Difference between?array_merge?and? ?in php?. For more information, please follow other related articles on the PHP Chinese website!