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The built-in json_encode() function in PHP puts everything on one line. For development and debugging, it can often be more useful to have properly indented, human-readable JSON that makes it easier to manually inspect the encoded data and check it for problems.

Here’s a function that lets you format/pretty-print any JSON string to be more readable:

/**
 * Indents a flat JSON string to make it more human-readable.
 *
 * @param string $json The original JSON string to process.
 * @param string $indentStr The string used for indenting nested structures. Defaults to 4 spaces.
 * @return string Indented version of the original JSON string.
 */
function pretty_json($json, $indentStr = '    ') {

	$result      = '';
	$level       = 0;
	$strLen      = strlen($json);
	$newLine     = "\n";
	$prevChar    = '';
	$outOfQuotes = true;
	$openingBracket = false;

	for ($i = 0; $i <= $strLen; $i++) {

		// Grab the next character in the string.
		$char = substr($json, $i, 1);

		// Add spaces before and after :
		if (($char == ':' && $prevChar != ' ') || ($prevChar == ':' && $char != ' ')) {
			if ($outOfQuotes) {
				$result .= ' ';
			}
		}

		// Are we inside a quoted string?
		if ($char == '"' && $prevChar != '\\') {
			$outOfQuotes = !$outOfQuotes;

			// If this character is the end of a non-empty element,
			// output a new line and indent the next line.
		} else if(($char == '}' || $char == ']') && $outOfQuotes) {
			$level --;
			if (!$openingBracket) {
				$result .= $newLine . str_repeat($indentStr, $level);
			}
		}

		// Add the character to the result string.
		$result .= $char;

		// If the last character was the beginning of a non-empty element,
		// output a new line and indent the next line.
		$openingBracket = ($char == '{' || $char == '[');
		if (($char == ',' || $openingBracket) && $outOfQuotes) {
			if ($openingBracket) {
				$level ++;
			}

			$nextChar = substr($json, $i + 1, 1);
			if ( ! ($openingBracket && ($nextChar == '}' || $nextChar == ']')) ) {
				$result .= $newLine . str_repeat($indentStr, $level);
			}
		}

		$prevChar = $char;
	}

	return $result;
}

Usage example:

$sampleData = array(
	'nested' => array(
		'a' => array(
			'b' => 1,
			'c' => null,
			'd' => array(1, 2, 3, 4)
		),
		'x' => array(1, 2, null, 'foo'),
		'y' => 3.14,
	),
	'special_chars' => ":,\'\"/\()[]&\r\n\t!",
	'escapes' => "\xc3\xa9",
	'empty_object' => new stdClass(),
	'empty_array' => array(),
);

$json = json_encode($sampleData);
echo "json_encode() output:\n", $json, "\n\n";

$pretty = pretty_json($json);
echo "Formatted JSON:\n", $pretty;

Output:

json_encode() output:
{"nested":{"a":{"b":1,"c":null,"d":[1,2,3,4]},"x":[1,2,null,"foo"],"y":3.14},"special_chars":":,\\'\"\/\\()[]&\r\n\t!","escapes":"\u00e9","empty_object":{},"empty_array":[]}

Formatted JSON:
{
    "nested" : {
        "a" : {
            "b" : 1,
            "c" : null,
            "d" : [
                1,
                2,
                3,
                4
            ]
        },
        "x" : [
            1,
            2,
            null,
            "foo"
        ],
        "y" : 3.14
    },
    "special_chars" : ":,\\'\"\/\\()[]&\r\n\t!",
    "escapes" : "\u00e9",
    "empty_object" : {},
    "empty_array" : []
}

Credit for the original JSON indentation function goes to Recursive Design. I have also made a few changes of my own:

• Add spaces around colons (based on this gist).
• Print empty arrays and objects on a single line.
• Configurable indentation (defaults to 4 spaces).