Why array_diff() gives Array to string conversion error?

I get array to string conversion error for the following line:

$diff = array_diff($stockist, $arr);

Here, $arr is an array decoded from a JSON file. Using the is_array() function I was able to verify that both parameters are arrays. Can someone point me the problem

$stockist = array();
while (!feof($file_handle)) {

    $line_of_text = fgetcsv($file_handle);
    $query = "SELECT * FROM reorderchart WHERE medicine = '"
        . trim($line_of_text[3])
        . "' ORDER BY medicine";
    $result = mysql_query($query);

    if (trim($line_of_text[2]) - trim($line_of_text[1]) <= 0) {
        
        while ($row = mysql_fetch_array($result)) {

            $file = "results.json";
            $arr = json_decode(file_get_contents($file),true);
            $pharmacy = trim($row['Medicine']);

            if (isset($stockist[$pharmacy])) {
                    
                $medicine = $stockist[$pharmacy];
                $medicine[] = trim($row['Stockist']);
                $stockist[$pharmacy] = $medicine;

            } else {

                $medicine = array();
                $medicine[] = trim($row['Stockist']);
                $stockist[$pharmacy] = $medicine;
            }
        }
    }
}
$diff = array();
$diff = array_diff_assoc($stockist,$arr);
ksort($diff);
foreach ($diff as $key => $value) {

    echo "<table align='center' border='1'>";
    echo "<tr><td align = 'center'> <font color = 'blue'> $key</td></tr>";
    
    foreach($value as $key1 => $value1) {

        echo "<tr><td align ='center'>$value1</td></tr><br>";
    }
    echo "</table>";
}

According to it:

php -r 'array_diff(array("a" => array("b" => 4)), array(1));'
PHP Notice:  Array to string conversion in Command line code on line 1
PHP Stack trace:
PHP   1. {main}() Command line code:0
PHP   2. array_diff() Command line code:1

One of your arrays is multidimensional.

array_diff only checks one dimension of a n-dimensional array. Of course you can check deeper dimensions by using array_diff($array1[0], $array2[0]);

Yes, the strict answer is because "One of your arrays is multidimensional."

Another useful note might be - depending on your needs of further parsing the actual differences - consider first testing your arrays with:

$diff = strcmp(json_encode($stockist), json_encode($arr));

or

$diff = strspn(json_encode($stockist) ^ json_encode($arr), "\0");

or

$diff = xdiff_string_diff(json_encode($stockist), json_encode($arr));

All these options will compare the entire array tree, not just the top level.

Since array_diff can only deals with one dimension, you can either:

• convert your multi-dimentional array into one dimension, e.g. by:

  • flattening a multidimensional array, e.g.:

      while ($arr) {
        list($key, $value) = each($arr); 
        is_array($value) ? $arr = $value : $out[$key] = $value;
        unset($arr[$key]);
      }
      // Now $out is flatten version of $arr.
    

  • serializing the array, e.g.:

      $results = array_diff(array_map('serialize',$a2), array_map('serialize',$a1)); 
    

• use custom recursive array_diff-like function,

According to PHP documentation for the function

> Note: Two elements are considered equal if and only if (string) $elem1 === (string) $elem2. In words: when the string representation is the same.

For more information refer to http://php.net/manual/en/function.array-diff.php

You can see in the array_diff() documentation that:

> Two elements are considered equal if and only if (string) $elem1 === > (string) $elem2. In other words: when the string representation is the > same.

So it looks like you can't use this function with multi dimensional array, or in fact any value that cannot be converted to a string. This is because the function will cast values to a string to do the comparison.

You can write your own function to recursively check arrays for a difference - in fact the following is from the comments of the docs linked above.

You can see the comment here.

function arrayRecursiveDiff($aArray1, $aArray2) { 
    $aReturn = array(); 
   
    foreach ($aArray1 as $mKey => $mValue) { 
        if (array_key_exists($mKey, $aArray2)) { 
            if (is_array($mValue)) { 
                $aRecursiveDiff = arrayRecursiveDiff($mValue, $aArray2[$mKey]); 
                if (count($aRecursiveDiff)) { $aReturn[$mKey] = $aRecursiveDiff; } 
            } else { 
                if ($mValue != $aArray2[$mKey]) { 
                    $aReturn[$mKey] = $mValue; 
                } 
            } 
        } else { 
            $aReturn[$mKey] = $mValue; 
        } 
    } 
   
    return $aReturn; 
}

Came across this one while working on a platform upgrade for PHP 7.3 from 5.3 today... Since I'm storing this value for use later on, I wanted to be sure I didn't wind up with a 'partially' serialized array that may break things downstream.

Thanks kenorb for getting me onto the right path (I upvoted ur answer). The following is working nicely for me.

Code:

$a1 = [
  'foo' => 'bar',
  'bar' => [
    'test' => 'test'
  ],
  'foobar' => 'fizzbuzz'
];

$a2 = [
  'foobar' => 'fizzbuzz',
  'herp' => [
    'derp' => 'herpderp'
  ]
];

$diff = array_diff(array_map('serialize', $a1), array_map('serialize', $a2));

$multidimensional_diff = array_map('unserialize', $diff);

print_r($multidimensional_diff);

Output:

Array
(
    [foo] => bar
    [bar] => Array
        (
            [test] => test
        )

)

This is my solution for a similar problem. I want to compare two associative arrays and return the changed values, but some of the elements are arrays. So if I use

array_diff_assoc

, it gives me "Array to string error". My function will also compare the elements which are arrays and if there's a difference, it will return the array element. It is still a work in progress and not tested extensively. For example:

public static $example1 = [
    'id' => 1,
    'status' => 2,
    'elements' => ['test', 'example'],
    'different' => ['old' => 5, 'new' => 9]
];

public static $example2 = [
    'id' => 1,
    'status' => 3,
    'elements' => ['test', 'example'],
    'different' => ['old' => 5, 'new' => 8]
];

public static function test(){
    die(var_dump(self::udiffAssoc(self::$example1, self::$example2)));
}

public static function udiffAssoc(array $array1, array $array2)
{
    $checkDiff = function ($a, $b) use (&$checkDiff) {
        if (is_array($a) && is_array($b)) {
            return array_udiff_assoc($a, $b, $checkDiff);
        } elseif (is_array($a)) {
            return 1;
        } elseif (is_array($b)) {
            return -1;
        } elseif ($a === $b) {
            return 0;
        } else {
            return $a > $b ? 1 : -1;
        }
    };
    return array_udiff_assoc($array1, $array2, $checkDiff);
}

if you run ::test it will return:

array(2) {
    ["status"]=>
    int(2)
    ["different"]=>
    array(2) {
    ["old"]=>
    int(5)
    ["new"]=>
    int(9)
  }
}

I've got the same error and found the following bug report for php:

https://bugs.php.net/bug.php?id=60198

> Some of the array_* functions that compare elements in multiple arrays > do so by (string)$elem1 === (string)$elem2. > > If $elem1 or $elem2 is an array, then the array to string notice is > thrown. > > Two examples of functions that can throw this are array_intersect() > and array_diff(). > > If these functions are not expected to take arrays with other arrays > as values, this should be mentioned on the documentation pages.

That report describes, why php throws an error on comparing a multi-dimensional array.

What about my solution:

$diff = strcmp(serialize($arr1), serialize($arr2))

We're getting cast to string warning because array_diff converts the elements to string before comparing... and we've passed ints, the key is to add a wrapper object since objects can be casted to string, then when comparison is done we're returning them back to ints as below

Or-

Your can just use a library https://github.com/voku/Arrayy#diffarray-array-static its have method called diff() can compare straight integer and other data types as well