今天在芒果小站看到一段代码，感觉挺不错的，于是转载过来。以下是原文。
将字符串形式的 JSON 数据格式化为缩进形式。通常使用 json_encode 转换出来的 JSON 串没有缩进，有这个方法就爽多了。
这里我默认使用了 tab 缩进，如果要改成空格，替换变量 $indentStr 即可。
/**
* Indents a flat JSON string to make it more human-readable.
* @param string $json The original JSON string to process.
* @return string Indented version of the original JSON string.
*/
function indent ($json) {
$result = '';
$pos = 0;
$strLen = strlen($json);
$indentStr = ' ';
$newLine = "";
$prevChar = '';
$outOfQuotes = true;
for ($i=0; $i<=$strLen; $i++) {
// Grab the next character in the string.
$char = substr($json, $i, 1);
// Are we inside a quoted string?
if ($char == '"' && $prevChar != '') {
$outOfQuotes = !$outOfQuotes;
// If this character is the end of an element,
// output a new line and indent the next line.
} else if(($char == '}' || $char == ']') && $outOfQuotes) {
$result .= $newLine;
$pos --;
for ($j=0; $j<$pos; $j++) {
$result .= $indentStr;
}
}
// Add the character to the result string.
$result .= $char;
// If the last character was the beginning of an element,
// output a new line and indent the next line.
if (($char == ',' || $char == '{' || $char == '[') && $outOfQuotes) {
$result .= $newLine;
if ($char == '{' || $char == '[') {
$pos ++;
}
for ($j = 0; $j < $pos; $j++) {
$result .= $indentStr;
}
}
$prevChar = $char;
}
return $result;
}