get value from imagebox into java script or in php

hi,
i have image box and some value coming on that image-box, what i want that,get that value. could it be possible to convert image in to string or character or any other way to get that value.

<tr>
<td height="30" valign="top" > </td>
<td valign="top">
<img id="imgid" name="img" src="CaptchaSecurityImages.php?width=100&height=40&characters=5" />
</td>
</tr>

i want this image value convert into string of in character in either by java script or by php.
or any alternate way to get the value of a image box.

if possible than pls revert ASAP.

thanks