PHP Cron作业json_encode错误

我有一个PHP文件，它正在运行一个简单的货币转换检查。当我通过浏览器运行它时，它运行得很好，但我的目标是构建一个cron。当我通过SSH运行脚本时：

php /path/to/file.php

我得到以下信息：

PHP Warning:  json_encode(): Invalid UTF-8 sequence in argument in /path/to/file.php on line 36

第36行：

fwrite($fh, json_encode($conversions));

其中$conversions是一个简单的一维数组

这是文件：

$conversions = array();
$currencies = json_decode(file_get_contents("/path/to/currencies.json"), true);
foreach($currencies as $cur=>$data){

    //make string to be put in API
    $string = "1USD=?".$data['code'];
    //Call Google API
    $google_url = "http://www.google.com/ig/calculator?hl=en&q=".$string;
    $ch = curl_init();  
    // set URL and other appropriate options  
    curl_setopt($ch, CURLOPT_URL, $google_url);  
    curl_setopt($ch, CURLOPT_HEADER, 0);  
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);     
    // grab URL and pass it to the browser    
    $result = curl_exec($ch);
    //Explode result to convert into an array
    $result = explode('"', $result);
    $converted_amount = explode(' ', $result[3]);
    $conversion = $converted_amount[0];
    //$conversion = $conversion * $amount;
    $conversions[$cur] = $conversion;
    if($conversion==0){ exit('0 Return Error'); }
    curl_close($ch);
}
$fh = fopen("/path/to/currency_conversions.json", 'w') or die("can't open file");
fwrite($fh, json_encode($conversions));
fclose($fh);