<?php

$array = [0 => ["number", null, "274", null, null, null, null, null, null, null],
		  1 => ["step", null, "4", null, null, null, null, null, null, null],
		  19 => ["details", null, null, null, null, null, null, null, null, null]];
		  
$new_array = array_combine(array_column($array, 0), array_column($array, 2));
echo json_encode($new_array);