это мое решение:
строка json $columns_validation = string(1736) "[{"colId":"N_ni","hide":true,"aggFunc":null,"width":136,"pivotIndex":null,"pinned":null,"rowGroupIndex":null},{"colId":"J_2_fait","hide":true,"aggFunc":null,"width":67,"pivotIndex":null,"pinned":null,"rowGroupIndex":null}]"
, поэтому я дважды использую json_decode:
$js_column_validation = json_decode($columns_validation);
$js_column_validation = json_decode($js_column_validation);
var_dump($js_column_validation);
и результат:
array(15) { [0]=> object(stdClass)#23 (7) { ["colId"]=> string(4) "N_ni" ["hide"]=> bool(true) ["aggFunc"]=> NULL ["width"]=> int(136) ["pivotIndex"]=> NULL ["pinned"]=> NULL ["rowGroupIndex"]=> NULL } [1]=> object(stdClass)#2130 (7) { ["colId"]=> string(8) "J_2_fait" ["hide"]=> bool(true) ["aggFunc"]=> NULL ["width"]=> int(67) ["pivotIndex"]=> NULL ["pinned"]=> NULL ["rowGroupIndex"]=> NULL }