$_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.

Practice

1. What does the $_FILES superglobal in PHP store?

easy

A. Database connection details

B. User session data

C. Information about files uploaded via an HTML form

D. Server environment variables

Solution

1. Step 1: Understand the purpose of $_FILES

   $_FILES is a special PHP variable that holds details about files uploaded through a form.

2. Step 2: Compare with other options

   User session data is stored in $_SESSION, database details are not stored here, and server variables are in $_SERVER.

3. Final Answer:

   Information about files uploaded via an HTML form -> Option C

4. Quick Check:

   $_FILES = Uploaded file info [OK]

Hint: Remember: $_FILES always relates to uploaded files [OK]

Common Mistakes:

• Confusing $_FILES with $_SESSION
• Thinking $_FILES holds server info
• Assuming $_FILES stores form text inputs

2. Which of the following is the correct way to access the name of an uploaded file with input name userfile?

easy

A. $_FILES['userfile']['filename']

B. $_FILES['userfile']['file']

C. $_FILES['userfile']['file_name']

D. $_FILES['userfile']['name']

Solution

1. Step 1: Recall the structure of $_FILES

   Each uploaded file's info is stored in an array with keys like 'name', 'type', 'tmp_name', 'error', and 'size'.

2. Step 2: Identify the correct key for the original filename

   The key for the original filename is 'name', not 'filename' or 'file_name'.

3. Final Answer:

   $_FILES['userfile']['name'] -> Option D

4. Quick Check:

   Filename key = 'name' [OK]

Hint: Use 'name' key to get original filename from $_FILES [OK]

Common Mistakes:

• Using 'filename' or 'file_name' instead of 'name'
• Confusing keys with other $_FILES elements
• Trying to access $_POST for file names

3. What will the following PHP code output if a file named 'photo.jpg' is uploaded successfully?

if ($_FILES['pic']['error'] === 0) {
    echo $_FILES['pic']['name'];
} else {
    echo 'Upload failed';
}

medium

A. photo.jpg

B. Upload failed

C. 0

D. Undefined index error

Solution

1. Step 1: Check the error code for the uploaded file

   An error code of 0 means the upload was successful with no errors.

2. Step 2: Output the original filename

   Since error is 0, the code echoes the original filename stored in $_FILES['pic']['name'], which is 'photo.jpg'.

3. Final Answer:

   photo.jpg -> Option A

4. Quick Check:

   Error 0 means success, so filename prints [OK]

Hint: Error 0 means success; print 'name' to get filename [OK]

Common Mistakes:

• Assuming error 0 means failure
• Printing error code instead of filename
• Not checking if 'pic' index exists

4. Identify the error in this PHP snippet handling file upload:

if ($_FILES['upload']['error'] == 0) {
    move_uploaded_file($_FILES['upload']['name'], 'uploads/' . $_FILES['upload']['name']);
    echo 'File saved';
} else {
    echo 'Error uploading';
}

medium

A. Not checking file size before moving

B. Using 'name' instead of 'tmp_name' in move_uploaded_file

C. Checking error with == instead of ===

D. Missing semicolon after echo statement

Solution

1. Step 1: Understand move_uploaded_file parameters

   The first parameter must be the temporary file path stored in tmp_name, not the original filename.

2. Step 2: Identify the incorrect usage

   The code incorrectly uses $_FILES['upload']['name'] as the source file, which is wrong and causes failure.

3. Final Answer:

   Using 'name' instead of 'tmp_name' in move_uploaded_file -> Option B

4. Quick Check:

   move_uploaded_file needs tmp_name as source [OK]

Hint: Use 'tmp_name' as source file in move_uploaded_file [OK]

Common Mistakes:

• Using original filename instead of temp file path
• Ignoring error codes before moving files
• Not validating file size or type

5. You want to save an uploaded file only if it is less than 2MB and has no errors. Which code snippet correctly checks these conditions before moving the file?

if (/* condition here */) {
    move_uploaded_file($_FILES['file']['tmp_name'], 'uploads/' . $_FILES['file']['name']);
    echo 'Upload successful';
} else {
    echo 'Upload failed';
}

hard

A. if ($_FILES['file']['error'] === 0 && $_FILES['file']['size'] < 2097152)

B. if ($_FILES['file']['error'] == 1 || $_FILES['file']['size'] > 2097152)

C. if ($_FILES['file']['error'] !== 0 && $_FILES['file']['size'] <= 2097152)

D. if ($_FILES['file']['error'] === 0 || $_FILES['file']['size'] > 2097152)

Solution

1. Step 1: Check for no upload errors

   The error code must be exactly 0 to confirm no errors during upload.

2. Step 2: Check file size limit

   The file size must be less than 2MB (2 * 1024 * 1024 = 2097152 bytes).

3. Step 3: Combine conditions correctly

   Both conditions must be true, so use logical AND (&&) with strict equality and less-than size check.

4. Final Answer:

   if ($_FILES['file']['error'] === 0 && $_FILES['file']['size'] < 2097152) -> Option A

5. Quick Check:

   Error 0 and size < 2MB = correct check [OK]

Hint: Use error === 0 and size < 2MB with && [OK]

Common Mistakes:

• Using || instead of && for conditions
• Checking error != 0 instead of === 0
• Confusing size units or limits