function array_count_values_of($value, $array) {
    $counts = array_count_values($array);
    return $counts[$value];
}

Not native, but come on, it's simple enough. ;-)

Alternatively:

echo count(array_filter($array, function ($n) { return $n == 6; }));

Or:

echo array_reduce($array, function (n) { return n == 6); }, 0);

Or:

echo count(array_keys($array, 6));

http://php.net/array_count_values

Did you attempt to research this at all?

 cv = array_count_values($arr);
 echo key];

That's how you get the count for ONE value.

In PHP7 you can use the NULL coalescing operator to simplify this code:

$a=['a','b','a','c','a','d'];

$output=[];

foreach ($a as $v) {
    $output[$v] = ($output[$v] ?? 0) + 1;
}

print_r($output);

Output:

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

Demo on 3v4l.org

To count matching occurance of a string in multidimensional array you will need to iterate over each array element and match the string and increment the count. Similiarly @Dor has suggested

$count = 0;
foreach ($array as $item) {
  if ($item->type === 'photo') {
    $count++;
  }
}

If you want achieve same in single dimensional array then It's pretty straightforward. You can use array_count_values PHP array function as explained below.

<?php
   $array = array(1, "test", 1, "php", "test");
   print_r(array_count_values($array));
?>

The above example will output:

Array
(
    [1] => 2
    [test] => 2
    [php] => 1
)

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