# cat j.php
<?php
$json = '[{
"pos": 1,
"uid": "44420575",
"rating": "14130271"
}, {
"pos": 2,
"uid": "24055142",
"rating": "1657068"
}, {
"pos": 3,
"uid": "29846213",
"rating": "1592081"
}]';
echo implode(',', array_column(json_decode($json, true) , 'uid'));
# php j.php
44420575,24055142,29846213