For PHP >=5.4.0, you can use JSON_PRETTY_PRINT option when using json_encode 
For PHP < 5.4.0, you can use code from a user contributed note in this page of PHP manual http://www.php.net/manual/en/function.json-encode.php#80339
<?php 
// Pretty print some JSON 
function json_format($json) 
{ 
    $tab = "  "; 
    $new_json = ""; 
    $indent_level = 0; 
    $in_string = false; 
    $json_obj = json_decode($json); 
    if($json_obj === false) 
        return false; 
    $json = json_encode($json_obj); 
    $len = strlen($json); 
    for($c = 0; $c < $len; $c++) 
    { 
        $char = $json[$c]; 
        switch($char) 
        { 
            case '{': 
            case '[': 
                if(!$in_string) 
                { 
                    $new_json .= $char . "\n" . str_repeat($tab, $indent_level+1); 
                    $indent_level++; 
                } 
                else 
                { 
                    $new_json .= $char; 
                } 
                break; 
            case '}': 
            case ']': 
                if(!$in_string) 
                { 
                    $indent_level--; 
                    $new_json .= "\n" . str_repeat($tab, $indent_level) . $char; 
                } 
                else 
                { 
                    $new_json .= $char; 
                } 
                break; 
            case ',': 
                if(!$in_string) 
                { 
                    $new_json .= ",\n" . str_repeat($tab, $indent_level); 
                } 
                else 
                { 
                    $new_json .= $char; 
                } 
                break; 
            case ':': 
                if(!$in_string) 
                { 
                    $new_json .= ": "; 
                } 
                else 
                { 
                    $new_json .= $char; 
                } 
                break; 
            case '"': 
                if($c > 0 && $json[$c-1] != '\\') 
                { 
                    $in_string = !$in_string; 
                } 
            default: 
                $new_json .= $char; 
                break;                    
        } 
    } 
    return $new_json; 
} 
?>