Example to explain how to query whether an image is converted in PHP

With the development of the Internet, image processing technology is also constantly improving, and there are more choices for image formats. Among them, common image formats include JPEG, PNG, GIF, etc. Different formats have different advantages and disadvantages for different needs. As a PHP developer, we often need to process images during the development process. One of the key issues is how to query whether the image is converted. This article will start from a beginner's perspective and combine it with real cases to introduce how to query whether an image has been converted.

First of all, let us first understand what image conversion is. Image conversion refers to the process of converting one image format into another image format. For example, we use software such as Photoshop to convert PNG format pictures into JPEG format pictures. There is not only one way to do this process, and the specific implementation depends on different software and requirements.

Next, let’s look at how to query whether the image is converted in PHP. In PHP, we can use the exif_imagetype() function to get the type of image. This function can determine a variety of common image formats, including JPEG, PNG, GIF, etc. The use of the function is very simple, just pass in the image path as a parameter. For example, the following code can get the type of an image.

$image_type = exif_imagetype('images/test.jpg');

The type returned by this function is not our common picture format, but a number. The corresponding relationship between numbers and picture formats is as shown in the following table:

##IMAGETYPE_JB2JBIG2 format

Type value	Image format
IMAGETYPE_GIF	GIF format
IMAGETYPE_JPEG	JPEG format
IMAGETYPE_PNG	PNG format
IMAGETYPE_BMP	BMP format
IMAGETYPE_WEBP	WEBP format
IMAGETYPE_TIFF_II	TIFF format (little endian)
IMAGETYPE_TIFF_MM	TIFF format (big endian)

After obtaining the type of image, we can judge whether the image is converted according to our own needs. For example, if we want to determine whether an image has been converted to JPEG format, we can use the following code:

$image_type = exif_imagetype('images/test.jpg');

if($image_type == IMAGETYPE_JPEG)
{
    echo '这张图片是JPEG格式的';
}
else
{
    echo '这张图片不是JPEG格式的';
}

It should be noted that the exif_imagetype() function only detects the image format, not the specific conversion Way. Therefore, in actual development, we need to choose different methods to query whether the image is converted according to actual needs.

Finally, let’s look at a practical case. On a certain website, the image format uploaded by users is only allowed to be in PNG format. However, in my testing, I found that some users uploaded images that were clearly in JPEG format, but the website still displayed normally. To check if anyone has somehow bypassed the upload limit, I'm going to query if these images have been converted. I used the exif_imagetype() function to get the image format, and got the following results:

图片路径                类型值
--------------------------------
images/test1.jpg    IMAGETYPE_JPEG
images/test2.jpg    IMAGETYPE_JPEG
images/test3.png    IMAGETYPE_PNG
images/test4.png    IMAGETYPE_PNG

It can be seen that the first two pictures are obviously in JPEG format, but they are saved in PNG format. After my inspection, I found that the website uses an image conversion software to convert JPEG format images to PNG format to bypass upload restrictions. In the end, I successfully discovered the vulnerability by querying whether the image was converted, and implemented security measures to improve the security of the website.

In short, querying whether an image is converted in PHP is a very important issue. We can use the exif_imagetype() function to obtain the type of image and choose different checking methods according to different needs. Especially in terms of ensuring website security, this issue needs to be taken more seriously.

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