问在使用imagettftext()时，如何处理字体文件不支持的字符？

EN

<?php
    require 'transliteration/JTransliteration.php';

    $input = 'ɦɪɲɣƙƨєʌɾ';
    echo JTransliteration::transliterate($input); // output: hIngk2ie^r

    $input = 'Хეλлఒ Wओრলद';
    echo JTransliteration::transliterate($input);

<?php
    $fontFile = 'arial.ttf';
    $charToCheck = 'ɣ';

    require_once 'php-font-lib-master/src/FontLib/Autoloader.php';

    use FontLib\Font;
    use FontLib\TrueType\Collection;


    $font = Font::load($fontFile);
    if ($font instanceof Collection) {
        $font = $font->getFont(0);
    }
    $subtable = null;
    foreach($font->getData("cmap", "subtables") as $_subtable) {
        if ($_subtable["platformID"] == 3 && $_subtable["platformSpecificID"] == 1) {
            $subtable = $_subtable;
            break;
        }
    }

    if (isset($subtable["glyphIndexArray"][ord_utf8($charToCheck)])) {
        $supported = 'Supported';
    } else {
        $supported = 'Not Supported';
    }

    echo "$charToCheck is $supported by font $fontFile";


    function ord_utf8($c) {
        $b0 = ord($c[0]);
        if ( $b0 < 0x10 ) {
            return $b0;
        }
        $b1 = ord($c[1]);
        if ( $b0 < 0xE0 ) {
            return (($b0 & 0x1F) << 6) + ($b1 & 0x3F);
        }
        return (($b0 & 0x0F) << 12) + (($b1 & 0x3F) << 6) + (ord($c[2]) & 0x3F);
    }

// utf-8 text
$text   = 'ɦɪɲɣƙƨєʌɾ';

// if text read from a file (for example)
// and the default locale (for most of western countries)
// is ISO-8859-1, you can simly convert it to
// utf-8 using:

//$text = utf8_encode($text);

$png    = imagecreatefrompng('/tmp/sample.png');
$color  = imagecolorallocate($png, 0, 0, 0);

// True type font that support UTF-8!!!!
$font   = '/usr/share/fonts/truetype/dejavu/DejaVuSans.ttf';

imagettftext($png, 50, 0, 50, 50, $color, $font, $text);
imagepng($png, '/tmp/test.png');