What is the difference between array_merge and array + array in PHP?

In PHP, both array_merge() and the + operator can combine arrays, but they handle duplicate keys differently. The + operator preserves values from the left array when keys conflict, while array_merge() overwrites duplicate string keys with values from the right array.

Using Array + Operator

The + operator keeps the original values when duplicate keys exist −

<?php
    $arr1 = array("p"=>"150", "q"=>"100", "r"=>"120", "s"=>"110");
    $arr2 = array("t"=>"115", "u"=>"103", "v"=>"105", "w"=>"125");
    var_dump($arr1 + $arr2);
?>

array(8) {
    ["p"]=>
    string(3) "150"
    ["q"]=>
    string(3) "100"
    ["r"]=>
    string(3) "120"
    ["s"]=>
    string(3) "110"
    ["t"]=>
    string(3) "115"
    ["u"]=>
    string(3) "103"
    ["v"]=>
    string(3) "105"
    ["w"]=>
    string(3) "125"
}

Using array_merge()

The array_merge() function combines arrays and overwrites duplicate string keys −

<?php
    $arr1 = array("p"=>"150", "q"=>"100", "r"=>"120", "s"=>"110");
    $arr2 = array("t"=>"115", "u"=>"110", "v"=>"105", "w"=>"100");
    var_dump(array_merge($arr1, $arr2));
?>

array(8) {
    ["p"]=>
    string(3) "150"
    ["q"]=>
    string(3) "100"
    ["r"]=>
    string(3) "120"
    ["s"]=>
    string(3) "110"
    ["t"]=>
    string(3) "115"
    ["u"]=>
    string(3) "110"
    ["v"]=>
    string(3) "105"
    ["w"]=>
    string(3) "100"
}

Key Difference with Duplicate Keys

Here's an example showing how they handle duplicate keys differently −

<?php
    $arr1 = array("a" => "first", "b" => "second");
    $arr2 = array("b" => "overwrite", "c" => "third");
    
    echo "Using + operator:
";
    print_r($arr1 + $arr2);
    
    echo "\nUsing array_merge():
";
    print_r(array_merge($arr1, $arr2));
?>

Using + operator:
Array
(
    [a] => first
    [b] => second
    [c] => third
)

Using array_merge():
Array
(
    [a] => first
    [b] => overwrite
    [c] => third
)

Conclusion

Use the + operator when you want to preserve original values for duplicate keys, and array_merge() when you want later values to overwrite earlier ones. Both methods handle numeric keys differently by reindexing them.