JSON Parsing and Empty Value Checking

<?php
header('Content-Type: text/plain');
$json_string = '[{"highest_education":"B.E ( Automobile Engineering )","occupation":"Job in Private Office","annual_income":""}]';

function method1($json_string) {
    $json_array = json_decode($json_string, true);
    if(isset($json_array['annual_income']) && !empty($json_array['annual_income'])) {
        // value is not null
    } else {
        // value is null or empty
    }
}

function method2($json) {
    $annual_income1='"annual_income":""';
    $annual_income2='"annual_income":null';
    if(strpos($json,$annual_income1) !== false || strpos($json,$annual_income2) !== false) {
        // value is null or empty
    }
}

$start = microtime(true);
for($i = 0; $i < 1000; $i++) {
    method1($json_string);
}
$end = microtime(true);
$elapsed_mehtod1 = round($end - $start, 6);
echo 'Method 1 used: ' . $elapsed_mehtod1 . 's' . PHP_EOL;

$start = microtime(true);
for($i = 0; $i < 1000; $i++) {
    method2($json_string);
}
$end = microtime(true);
$elapsed_mehtod2 = round($end - $start, 6);
echo 'Method 2 used: ' . $elapsed_mehtod2 . 's' . PHP_EOL;