how to convert multidimensional array to single dimensional array in php

$records = array(
    array(
        'id' => 2135,
        'first_name' => 'jessica',
        'last_name' => 'ram',
    ),
     'id'=>2134,
    array(
        'id' => 5342,
        'first_name' => 'Ram',
        'last_name' => 'ranghuvanshi',
    ) 
);

 
 
$first_names = array_column($records, 'first_name');
print_r($first_names);

$array=array(
	"data",
 array("is","money")
 );
 $flat_array;
 function flat_array($array){
	global $flat_array;
	foreach($array as $value){
		if(is_array($value)){
		 flat_array($value); 
		}else {
			$flat_array[]=$value;
			 
		}
		
	}
	 
 }
 flat_array($array);
 print_r($flat_array);

$iterator = new RecursiveIteratorIterator(
    new RecursiveArrayIterator($records)  
);
var_dump( $iterator); PHP_EOL;
foreach ($iterator as $key => $leaf) {
    echo " $leaf", PHP_EOL;
}