先附上一自定义函数代码

function autowrap($fontsize, $angle, $fontface, $string, $width) {
// 这几个变量分别是 字体大小, 角度, 字体名称, 字符串, 预设宽度 
    $content = "";
 
    // 将字符串拆分成一个个单字 保存到数组 letter 中
    for ($i=0;$i < mb_strlen($string);$i++) {
        $letter[] = mb_substr($string, $i, 1);
    }
 
    foreach ($letter as $l) {
        $teststr = $content." ".$l;
        $testbox = imagettfbbox($fontsize, $angle, $fontface, $teststr);
        // 判断拼接后的字符串是否超过预设的宽度
        if (($testbox[2] > $width) && ($content !== "")) {
            $content .= "\n";
        }
        $content .= $l;
    }
    $content = mb_convert_encoding($content, "UTF-8", "GB2312");
    return $content;
}

能根据给的参数条件自动换行。。。
但是换行之后可能会出现乱码。

<?php
header("Content-type:text/html;charset=UTF-8");

$string = '曾任设法435发的防撒旦发射大法师发送';//必须为GBK  
echo autowrap(12,0,'simhei.ttf',$string,200);

function autowrap($fontsize, $angle, $fontface, $string, $width) {
	// 这几个变量分别是 字体大小, 角度, 字体名称, 字符串, 预设宽度
	$content = "";

	$letter = singleWordToArray($string);
	foreach ($letter as $l) {
		$teststr = $content." ".$l;
		$testbox = imagettfbbox($fontsize, $angle, $fontface, $teststr);
		// 判断拼接后的字符串是否超过预设的宽度
		if (($testbox[2] > $width) && ($content !== "")) {
			$content .= "\n";
		}
		$content .= $l;
	}
	$content = mb_convert_encoding($content, "UTF-8", "GB2312");
	return $content;
}

function singleWordToArray($str,$spaces=true){
	$n = 0;
	$len = strlen($str);
	$tmp = array();
	while ($n<$len){
		if(ord(substr($str,$n,1))>0xa0){
			$tmp[] = substr($str,$n,2);
			$n++;
		}else{
			$char = trim(substr($str,$n,1));
			if($spaces)
			{
				$tmp[] = $char;
			}else{
				if ($char!='') $tmp[] = $char;
			}
		}
		$n++;
	}//end while
	return $tmp;
}