


what is the difference between php array_merge and the operator
Jul 11, 2025 am 02:11 AMThe key difference between array_merge() and operators when merging arrays is the way keys are processed and values ??are overwritten. 1. array_merge() will re-index the numeric keys and retain the string keys. The key value of the same name in the subsequent array will overwrite the previous one; 2. The operator will retain all keys. When encountering key conflicts, the value of the left array is retained, and the value of the right array is ignored. Therefore, if you need to allow overrides and don't mind the number keys being rearranged, use array_merge(); if you need to keep the original key value and avoid overrides, use operators.
In PHP, both array_merge()
and the
operator are used to combine arrays, but they behave quite differently — especially when dealing with overlapping keys. Here's how they different and when you might want to use each one.

How array_merge()
works
When you use array_merge()
, PHP combines arrays and reindexes numeric keys starting from zero, while preserving string keys. If there are overlapping keys (especially string keys), the later array's value will overwrite the earlier one.

Example:
$a = ['a' => 1, 'b' => 2]; $b = ['b' => 3, 'c' => 4]; $result = array_merge($a, $b); // Results: ['a' => 1, 'b' => 3, 'c' => 4]
- Numeric keys get reindexed.
- String keys are preserved.
- Duplicate string keys in later arrays replace earlier ones.
Use this when:

- You want to merge arrays and allow overwriting of duplicate keys.
- You're OK with numeric keys being renumbered.
How the
operator works
The
operator merges two or more arrays by preserving all keys , and if there's a key collision, the value from the leftmost array wins — meaning duplicates in the right-hand array are ignored.
Example:
$a = ['a' => 1, 'b' => 2]; $b = ['b' => 3, 'c' => 4]; $result = $a $b; // Results: ['a' => 1, 'b' => 2, 'c' => 4]
- Keys are not reindexed (both numeric and string).
- In case of key conflicts, the first occurrence is kept.
- The order matters — left side takes priority.
Use this when:
- You want to preserve all original values, even if there are duplicates.
- You need to "fallback" data from another array without overriding existing keys.
Key differences summarized
Here's a quick breakdown:
-
Key handling:
-
array_merge()
reindexes numeric keys and replaces duplicate string keys. -
-
-
Overwrite behavior:
-
array_merge()
lets newer values ??overwrite older ones. -
-
-
Return type:
- Both return a new array and don't modify the originals.
So depending on whether you care about preserving keys or allowing overwrites, you'll pick one or the other.
Basically that's it.
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