$_FILES for file uploads in PHP - Step-by-Step Execution

Concept Flow - $_FILES for file uploads

User selects file in form

↓

Form submits with enctype="multipart/form-data"

↓

PHP receives request

↓

$_FILES array is populated

↓

Access $_FILES['input_name'

↓

Process file: check errors, move uploaded file

↓

File saved or error handled

This flow shows how a file selected by the user in a form is sent to PHP, which fills the $_FILES array to let you access file info and handle the upload.

Execution Sample

PHP

<?php
if ($_FILES['myfile']['error'] === 0) {
  move_uploaded_file($_FILES['myfile']['tmp_name'], 'uploads/' . $_FILES['myfile']['name']);
  echo 'Upload successful';
} else {
  echo 'Upload failed';
}
?>

This code checks if the file uploaded without error, then moves it from temporary storage to the 'uploads' folder and prints success or failure.

Execution Table

Step	Action	$_FILES['myfile'] Content	Result/Output
1	User selects file and submits form	Empty before submit	No output
2	PHP receives request, $_FILES populated	{"name":"photo.jpg","type":"image/jpeg","tmp_name":"/tmp/php1234","error":0,"size":34567}	No output
3	Check error == 0	error=0	Condition true, proceed
4	Call move_uploaded_file(tmp_name, 'uploads/photo.jpg')	File moved to uploads/photo.jpg	File saved
5	Print success message	N/A	Upload successful
6	If error not 0	N/A	Upload failed (not reached here)

💡 Execution stops after printing success or failure message

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
$_FILES['myfile']['error']	undefined	0	0	0	0
$_FILES['myfile']['tmp_name']	undefined	/tmp/php1234	/tmp/php1234	moved	moved
Output message	empty	empty	empty	empty	Upload successful

Key Moments - 3 Insights

Why do we check if $_FILES['myfile']['error'] is 0 before moving the file?

Because error 0 means the upload succeeded. If we skip this check, we might try to move a file that wasn't uploaded properly, causing errors. See execution_table row 3.

What is the purpose of $_FILES['myfile']['tmp_name']?

It is the temporary location where PHP stores the uploaded file before you move it. You must use this path to move the file to a permanent place. See execution_table row 4.

Why must the form use enctype="multipart/form-data"?

Without this encoding, the file data won't be sent properly, so $_FILES will be empty. This is essential for file uploads.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the value of $_FILES['myfile']['error']?

A 0

B 1

C Undefined

D null

💡 Hint

Check the '$_FILES["myfile"] Content' column at step 3 in execution_table

At which step does the file get moved to the 'uploads' folder?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look for the action 'Call move_uploaded_file' in execution_table

If $_FILES['myfile']['error'] was 1 instead of 0, what would be the output?

A Upload successful

B Upload failed

C No output

D File moved

💡 Hint

See execution_table row 6 for error not 0 scenario

Concept Snapshot

PHP $_FILES holds info about uploaded files.
Check $_FILES['input_name']['error'] == 0 before processing.
Use $_FILES['input_name']['tmp_name'] to move file.
Form must have enctype="multipart/form-data".
Move file with move_uploaded_file() to save it.
Always handle errors to avoid issues.

Full Transcript

When a user selects a file in a form and submits it, the form must use the encoding type multipart/form-data. PHP then fills the $_FILES array with details about the uploaded file, including its name, type, temporary location, error code, and size. We check if the error code is zero, meaning the upload succeeded. Then we move the file from the temporary location to a permanent folder using move_uploaded_file(). Finally, we print a success or failure message. This process ensures safe handling of file uploads in PHP.