In your code

$arr[$i]=$arr[$i]/1000;
$arr1[$i]=$arr[$i];

Your $arr1 is a collection of floats:

Array
(
    [0] => 1.1
    [1] => 3.15
    [2] => 4.4
    [3] => 4.4
    [4] => 5.17
    [5] => 7.45
    [6] => 7.45
    [7] => 7.45
    [8] => 8.23
)

Hence the Can only count STRING and INTEGER values.

You can round it or do something else like cast it to a string etc...

$arr[$i]=$arr[$i]/1000;
$arr1[$i]=(string)$arr[$i];

Also

function sd_square($x, $total) { return pow($x - $mean,2); }

The var $mean is undefined here.

Making those changes:

<?php

echo "Welcome to my project".'<br>'.'<br>'; 
$arr=array(1100,3150,4400,4400,5170,7450,7450,7450,8230 );
$arr1=[]; //<--- define this if all are < 100 its undefined
for($i=0; $i<=8; $i++){
    if ($arr[$i]<100) {  //<-- clean up formatting.
        $arr[$i]=$arr[$i];
    }else{
        $arr[$i]=$arr[$i]/1000;
        $arr1[$i]=(string)$arr[$i]; //<-- cast to string
    }
}

function calculate($arr, $output){

        switch($output){
            case 'mean':
                $count = count($arr)+1;
                $sum = array_sum($arr);
                $total = $sum / $count;
            break;
            case 'median':
                rsort($arr);
                $middle = (count($arr) / 2)+1;
                $total = $arr[$middle-1];
            break;
            case 'mode':
                $v = array_count_values($arr); 
                arsort($v); 
                foreach($v as $k => $v){$total = $k; break;}

            break;

        }
        return $total;
    }

function sd_square($x, $total) { return pow($x - $total,2); } //<--changed to $total
function sd($arr) {
    return sqrt(array_sum(array_map("sd_square", $arr, array_fill(0,count($arr), (array_sum($arr) / count($arr)) ) ) ) / (count($arr)-1) );
}

   if (isset($_POST['select'])) {
    someFunction();
  }

echo '  '.'<br>';
echo "Values: ";
echo json_encode($arr).'<br>';
echo 'Mean: '.calculate($arr, 'mean').'<br>';
echo 'Median: '.calculate($arr, 'median').'<br>';
echo 'Mode: '.calculate($arr1, 'mode').'<br>';
echo "Standart Derivation: ".sd($arr);

Output

Welcome to my project

Values: [1.1,3.15,4.4,4.4,5.17,7.45,7.45,7.45,8.23]
Mean: 4.88
Median: 5.17
Mode: 7.45
Standart Derivation: 2.4035743059961

Sandbox

I think you should not index by the prices, after all from a math point of view 5.0 and 5.00 does not make difference at all.

If you are obtaining values from a database you will get strigs everywhere. So you will have to cast (int)$key for the keys.

And in your foreach you are changing a temporary variable. $key exists only for the current iteration of the loop you will want to declare it as:

foreach($list_b_prices as &(int)value){
    if($price_count[$key] >= 2){
        $key . "0";
    }
}

Note the ampersand and the casting to integer. Although i'm not sure which will come first. But again: I think indexing by some different value shall give you a better result.

you have null values in your array. you have to replace them before working with array_count_values like this:

$x = array('s'=>'ss', 'a',4 , 'sss' => null);

$ar = array_replace($x,array_fill_keys(array_keys($x, null),''));

$v = array_count_values($ar);

var_dump($v);

which will result:

array (size=4)
  'ss' => int 1
  'a' => int 1
  4 => int 1
  '' => int 1

The issue is right here in the source data:

{
    "code": null
},

Adding an array_filter() to the mix will resolve it:

$json = array_count_values(array_filter(array_column($json, "code")));

If you don't pass a callable to array_filter(), it will just filter out elements that are falsy, like null.

You can't do it. The function array_count_values works only for string or integer:

Throws E_WARNING for every element which is not string or integer.

For complex objects like this, you need to take a different method. Since your PHP doesn't output the warning messages, it is not getting displayed. If you want, you can do something like a transformation.

$finalItems = array();
// Loop through the items of the object and push the names into the array.
// ... Logic ...
// And finally do:
array_count_values($finalItems);

$array = array("Kyle","Ben","Sue","Phil","Ben","Mary","Sue","Ben");
$counts = array_count_values($array);
echo $counts['Ben'];

You seem to have duplicate data, Assuming you can transform your array to something like this:

array(
[0]=>'SI',
[1]=>'SI',
[2]=>'NO',
[3]=>'SI',
[4]=>'SI',
);

Solution 1: You can use array_count_values().

$array = array([0]=>'SI', [1]=>'SI', [2]=>'NO', [3]=>'SI', [4]=>'SI',);
print_r(array_count_values($array));

This will output something like this:

Array
(
    [SI] => 4
    [NO] => 1
)

https://www.php.net/manual/en/function.array-count-values.php

Solution 2:

If you can't change your data, you can use array_map().

function array_count_values_rep($arr)
{
    return $arr['rep_onoff'];
}

$arr = array(
    array(
        0 => 'NO',
        'rep_onoff' => 'NO',
    ),
    array(
        0 => 'SI',
        'rep_onoff' => 'SI',
    ),
    array(
        0 => 'SI',
        'rep_onoff' => 'SI',
    ),
    array(
        0 => 'SI',
        'rep_onoff' => 'SI',
    ),
    array(
        0 => 'NO',
        'rep_onoff' => 'NO',
    ),
    array(
        0 => 'SI',
        'rep_onoff' => 'SI',
    ),
    array(
        0 => 'SI',
        'rep_onoff' => 'SI',
    ),
);
$result = array_map('array_count_values_rep', $arr);
print_r(array_count_values($result));

Which will output:

Array ( [NO] => 2 [SI] => 5 )

https://www.php.net/manual/en/function.array-map.php