- array_sum: documentation ( source)
- json_decode: documentation ( source)
- array_column: documentation ( source)
<?php
// these are subarrays containing 1 element each
$json = '[{"rate":3},{"rate":4},{"rate":3},{"rate":5}]';
// decode it to an array
$array = json_decode($json, true);
// extract the "rate" column values from each subarray and sum them
echo array_sum(array_column($array, 'rate'));