這個方法我也有查到~
可是就如同我PS3說的..
因為某種原因無法設定字型
那個原因就是我用TCPDF將傳給我的HTML字串轉成PDF檔..
而TCPDF字體的設定是
$pdf->setFont('cid0ct', '', 14);
其中..cid0ct是TCPDF內建字型
檔案是PHP不是ttf檔

那這個呢?
http://stackoverflow.com/questions/25208064/tcpdf-how-to-use-getstringwidth-when-text-is-bold

原始碼:

	/**
	 * Returns the length of a string in user unit. A font must be selected.<br>
	 * @param $s (string) The string whose length is to be computed
	 * @param $fontname (string) Family font. It can be either a name defined by AddFont() or one of the standard families. It is also possible to pass an empty string, in that case, the current family is retained.
	 * @param $fontstyle (string) Font style. Possible values are (case insensitive):<ul><li>empty string: regular</li><li>B: bold</li><li>I: italic</li><li>U: underline</li><li>D: line-through</li><li>O: overline</li></ul> or any combination. The default value is regular.
	 * @param $fontsize (float) Font size in points. The default value is the current size.
	 * @param $getarray (boolean) if true returns an array of characters widths, if false returns the total length.
	 * @return mixed int total string length or array of characted widths
	 * @author Nicola Asuni
	 * @public
	 * @since 1.2
	 */
	public function GetStringWidth($s, $fontname='', $fontstyle='', $fontsize=0, $getarray=false) {
		return $this->GetArrStringWidth(TCPDF_FONTS::utf8Bidi(TCPDF_FONTS::UTF8StringToArray($s, $this->isunicode, $this->CurrentFont), $s, $this->tmprtl, $this->isunicode, $this->CurrentFont), $fontname, $fontstyle, $fontsize, $getarray);
	}

剛剛試完~確認可以使用~
不過還有兩個問題待解決~

1. 是單位問題，因為我的PDF都是Table組成的，裡面的表格寬度數值設定與GetStringWidth回傳的數值設定不一樣，例如:我其中一個表格width=517，但內容的寬度數值回傳145.8222222....(內容為了測試已故意用寬度最窄的i填滿表格寬了)，這個問題我應該還能掌握，只是不知道為什麼會這樣~= =
2. 是GetStringWidth這個函式因為會被使用很多次，因為我目的是要判斷每個表格內的內容寬度(包含自動換行後的總寬)，所以我將它放在function中(因為我表格內容本身就是用function去跑的)，但是卻無法執行，放在外層就可以執行，這個我無法掌握，研究中，求解~XD

getStringWidth呼叫getArrStringWidth:

	/**
	 * Returns the string length of an array of chars in user unit or an array of characters widths. A font must be selected.<br>
	 * @param $sa (string) The array of chars whose total length is to be computed
	 * @param $fontname (string) Family font. It can be either a name defined by AddFont() or one of the standard families. It is also possible to pass an empty string, in that case, the current family is retained.
	 * @param $fontstyle (string) Font style. Possible values are (case insensitive):<ul><li>empty string: regular</li><li>B: bold</li><li>I: italic</li><li>U: underline</li><li>D: line through</li><li>O: overline</li></ul> or any combination. The default value is regular.
	 * @param $fontsize (float) Font size in points. The default value is the current size.
	 * @param $getarray (boolean) if true returns an array of characters widths, if false returns the total length.
	 * @return mixed int total string length or array of characted widths
	 * @author Nicola Asuni
	 * @public
	 * @since 2.4.000 (2008-03-06)
	 */
	public function GetArrStringWidth($sa, $fontname='', $fontstyle='', $fontsize=0, $getarray=false) {
		// store current values
		if (!TCPDF_STATIC::empty_string($fontname)) {
			$prev_FontFamily = $this->FontFamily;
			$prev_FontStyle = $this->FontStyle;
			$prev_FontSizePt = $this->FontSizePt;
			$this->SetFont($fontname, $fontstyle, $fontsize, '', 'default', false);
		}
		// convert UTF-8 array to Latin1 if required
		if ($this->isunicode AND (!$this->isUnicodeFont())) {
			$sa = TCPDF_FONTS::UTF8ArrToLatin1Arr($sa);
		}
		$w = 0; // total width
		$wa = array(); // array of characters widths
		foreach ($sa as $ck => $char) {
			// character width
			$cw = $this->GetCharWidth($char, isset($sa[($ck + 1)]));
			$wa[] = $cw;
			$w += $cw;
		}
		// restore previous values
		if (!TCPDF_STATIC::empty_string($fontname)) {
			$this->SetFont($prev_FontFamily, $prev_FontStyle, $prev_FontSizePt, '', 'default', false);
		}
		if ($getarray) {
			return $wa;
		}
		return $w;
	}

其中, $getarray可做為除錯用, 看看個別字元的寬度. 另外, 您可以在此函數中嵌進除錯碼, 看看發生什麼事. 或使用除錯器看看.

getArrStringWidth呼叫getCharWidth

	/**
	 * Returns the length of the char in user unit for the current font considering current stretching and spacing (tracking).
	 * @param $char (int) The char code whose length is to be returned
	 * @param $notlast (boolean) If false ignore the font-spacing.
	 * @return float char width
	 * @author Nicola Asuni
	 * @public
	 * @since 2.4.000 (2008-03-06)
	 */
	public function GetCharWidth($char, $notlast=true) {
		// get raw width
		$chw = $this->getRawCharWidth($char);
		if (($this->font_spacing < 0) OR (($this->font_spacing > 0) AND $notlast)) {
			// increase/decrease font spacing
			$chw += $this->font_spacing;
		}
		if ($this->font_stretching != 100) {
			// fixed stretching mode
			$chw *= ($this->font_stretching / 100);
		}
		return $chw;
	}

建議您追蹤TCPDF的原始碼去檢查怎麼計算各寬度的.

老大！！
我找到function不能執行GetStringWidth的原因了~
原本是這麼做的..

function test($text) {
    $textwidth = $pdf->GetStringWidth('Text Test', 'cid0ct', '', 14);
    //...以下省略...
}

我猜會不會是$pdf的關係，於是我這麼做..

function test($text) {
    $pdf = new MYPDF(PDF_PAGE_ORIENTATION, PDF_UNIT, PDF_PAGE_FORMAT, true, 'UTF-8', false);
    $textwidth = $pdf->GetStringWidth('Text Test', 'cid0ct', '', 14);
    //...以下省略...
}

雖然我function外層也有同一行在做定義$pdf
但好像到function就必須重新定義一次
不過我有個問題...
因為每個表格都會跑一次function
每次都new一個新的...會不會有問題~
(雖然function執行完後這個內部$pdf會不見..)