<?php
 
$records = [
	0 => [
		'first_name' => 'abc',
		'last_name' => 'cba',
		'age' => '1'
	],
	1 => [
		'first_name' => 'def',
		'last_name' => 'fed',
		'age' => '2'
	],
	2 => [
		'first_name' => 'ghi',
		'last_name' => 'ihg',
		'age' => '3'
	]
];
 
// Using the $records array from Example #1
//$firstNames = array_column($records, 'first_name', 'first_name');
$firstNames = array_column($records, 'first_name');
 
var_dump(json_encode($firstNames));

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