Difference between array_merge() and array_combine().

🔹 array_merge()

• Definition: Merges one or more arrays into a single array.
• Behavior:
  • Numeric keys are reindexed (0,1,2,…).
  • String keys are preserved, and if the same key exists in multiple arrays, the later one overwrites the earlier one.

Example:

$a = ["red", "green"];
$b = ["blue", "yellow"];

print_r(array_merge($a, $b));

Output:

Array
(
    [0] => red
    [1] => green
    [2] => blue
    [3] => yellow
)

With duplicate string keys:

$a = ["color" => "red", "shape" => "circle"];
$b = ["color" => "blue", "size" => "large"];

print_r(array_merge($a, $b));

Output:

Array
(
    [color] => blue    // overwritten
    [shape] => circle
    [size] => large
)

✅ Use array_merge() when you want to combine values of multiple arrays.

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🔹 array_combine()

• Definition: Creates an array by using one array for keys and another array for values.
• Condition: Both arrays must have the same number of elements, otherwise it throws an error.

Example:

$keys = ["a", "b", "c"];
$values = [1, 2, 3];

print_r(array_combine($keys, $values));

Output:

Array
(
    [a] => 1
    [b] => 2
    [c] => 3
)

⚠️ If the sizes don’t match:

$keys = ["a", "b"];
$values = [1, 2, 3];
print_r(array_combine($keys, $values));

Error:

Warning: array_combine(): Both parameters should have an equal number of elements

✅ Use array_combine() when you want to map keys to values explicitly.

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🔑 Key Differences

Feature	array_merge()	array_combine()
Purpose	Combines multiple arrays	Creates new array from keys & values
Keys handling	Numeric keys reset; String keys preserved	First array = keys, Second array = values
Overwriting	Later arrays overwrite same string keys	No overwrite (direct mapping)
Size restriction	Any number of arrays	Both arrays must have equal length
Example Use Case	Merging config arrays, appending lists	Creating dictionary-like structures

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👉 In short:

• Use array_merge() when you want to join arrays.
• Use array_combine() when you want to pair keys with values.