How to add key to json/php arrays

Try this

The array_combine() function creates an array by using the elements from one "keys" array and one "values" array.

<?php
$array1 = array('id','name1','name2','name3','name4');

$array2 = [["1","1","12","34","text1"],["2","1","56","78","text2"]];
$result = array();
//echo "<pre>";
foreach ($array2 as $value) {
    $result[] = array_combine($array1,$value);
}
//print_r($result);
echo json_encode($result);
 ?>

NOTE: Here array elements of $array1 and array elements of $value must be same otherwise it returns FALSE.

You can use array_combine() to create an array from two other using one for keys and the other for values (Note both must have the same length!)

$array1 = array('id','name1','name2','name3','name4');
$array2 = [["1","1","12","34","text1"],["2","1","56","78","text2"]];

foreach ($array2 as &$set) {
    $set = array_combine($array1, $set);
}
unset($set);

echo json_encode($array2);

Here we pass each inner array into the foreach by reference (the & in &$set) so we can modify it directly without having to create a third result array, we also unset it after the loop.

You should use the definition of array(), which is:

array(
    key  => value,
    key2 => value2,
    key3 => value3,
    ...
    )

You have your data in each element of array2, you can iterate it, and create a new array for each element using the constructor that I have posted. So, an example will be:

array('id'-->array2[i][j],'name1'=>array[i][j+1],...)

You can use functional iteration to avoid temporary global variables.

Code: (Demo)

$keys = ['id', 'name1', 'name2', 'name3', 'name4'];

$rows = [
    ["1", "1", "12", "34", "text1"],
    ["2", "1", "56", "78", "text2"]
];

echo json_encode(
         array_map(
             function($row) use ($keys) {
                 return array_combine($keys, $row);
             },
             $rows
         )
     );

Or with PHP7.4 and higher enjoy arrow function syntax:

echo json_encode(
         array_map(
             fn($row) => array_combine($keys, $row),
             $rows
         )
     );

Ultimately, though, if this data is coming from any kind of sql source, then you should be managing the column names in the SELECT clause and then php simply creates associative subarrays as you fetch and push the data into your result array.