How does one enable users to select images in the file domain and then display them dynamically?

How can users select images in the file domain and then display them dynamically? Many websites on the Internet have the upload Avatar function. after selecting a file in the file domain, it can be displayed on the webpage before submission. but how can this problem be achieved? In other words, images on the Internet should be server resources. but how can this function be implemented? Please give a detailed explanation or how to make sure that the user selects an image in the file domain and then displays it dynamically?
Many websites on the Internet have the upload Avatar function. after selecting a file in the file domain, it can be displayed on the webpage before submission. but how can this problem be achieved?

In other words, images on the Internet should be server resources. but how can this function be implemented?

Please give us a detailed explanation or detailed PHP code !!

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Discussion

No, for example, renrenren is actually uploaded and displayed to the user. you can manually take part of the image and save and publish it. However, the upload is an ajax upload, so it does not seem to have been uploaded. Although the upload is complete, it is not saved until the user clicks "save" and then saves it in the background, but the image is already in the background. you can see the url of the image.

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After jq is selected, you can view the image without uploading it.
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AlphaImageLoader in IE
FileReader for other browsers with html features