File existence and info checks in PHP - Step-by-Step Execution

Concept Flow - File existence and info checks

Start

↓

Check if file exists

Yes↓

Get file info

↓

Display info

↓

End

No↓

Display 'File not found'

↓

End

The program first checks if a file exists. If yes, it gets and shows file details. If not, it shows a 'File not found' message.

Execution Sample

PHP

<?php
$filename = 'example.txt';
if (file_exists($filename)) {
  echo filesize($filename);
} else {
  echo 'File not found';
}
?>

This code checks if 'example.txt' exists and prints its size or a not found message.

Execution Table

Step	Action	Evaluation	Result
1	Set $filename = 'example.txt'	Assign string	$filename = 'example.txt'
2	Check file_exists('example.txt')	Returns true if file exists	true
3	Since true, call filesize('example.txt')	Returns file size in bytes	1234
4	Print file size	Output file size	1234
5	End of script	No more code	Script ends

💡 Execution stops after printing file size because file exists.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
$filename	undefined	'example.txt'	'example.txt'	'example.txt'	'example.txt'
file_exists result	undefined	undefined	true	true	true
filesize result	undefined	undefined	undefined	1234	1234

Key Moments - 2 Insights

Why does the code check file_exists before calling filesize?

Because calling filesize on a non-existing file causes an error. The check prevents this by ensuring the file is there first (see step 2 and 3 in execution_table).

What happens if the file does not exist?

The else branch runs, printing 'File not found' instead of trying to get file info (not shown in this example but implied by the else block).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of $filename after step 1?

A 'example.txt'

B undefined

C 1234

D true

💡 Hint

Check the variable_tracker row for $filename at 'After Step 1'

At which step does the code decide the file exists?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'file_exists result' in variable_tracker and execution_table step 2

If the file did not exist, what would the output be?

A The file size in bytes

B An error message from filesize

C 'File not found'

D No output

💡 Hint

The else branch prints 'File not found' when file_exists returns false

Concept Snapshot

PHP file existence and info checks:
- Use file_exists('filename') to check if a file is present.
- Use filesize('filename') to get file size in bytes.
- Always check file_exists before filesize to avoid errors.
- Use if-else to handle file found or not found cases.

Full Transcript

This example shows how PHP checks if a file exists using file_exists. If the file is there, it gets the file size with filesize and prints it. If not, it prints 'File not found'. The execution table traces each step: setting the filename, checking existence, getting size, and printing. The variable tracker shows how variables change. Key moments explain why the existence check is important to avoid errors. The quiz tests understanding of variable values and flow decisions.

Practice

1. Which PHP function checks if a file or directory exists at a given path?

easy

A. filesize()

B. is_file()

C. file_exists()

D. is_readable()

Solution

1. Step 1: Understand the purpose of file_exists()

   The function file_exists() returns true if the file or directory exists at the specified path.

2. Step 2: Compare with other functions

   Functions like is_file() check if the path is a file, filesize() returns size, and is_readable() checks permissions, but none check existence alone.

3. Final Answer:

   file_exists() -> Option C

4. Quick Check:

   Check existence = file_exists() [OK]

Hint: Use file_exists() to check any file or directory presence [OK]

Common Mistakes:

• Confusing is_file() with existence check
• Using filesize() to check existence
• Assuming is_readable() means file exists

2. Which of the following is the correct syntax to check if a path is a directory in PHP?

easy

A. is_directory($path);

B. is_dir($path);

C. file_is_dir($path);

D. dir_exists($path);

Solution

1. Step 1: Recall the correct function name

   PHP uses is_dir() to check if a path is a directory.

2. Step 2: Verify syntax correctness

   The function is called as is_dir($path); other options are invalid function names.

3. Final Answer:

   is_dir($path); -> Option B

4. Quick Check:

   Directory check = is_dir() [OK]

Hint: Remember: is_dir() checks directories, not is_directory() [OK]

Common Mistakes:

• Using non-existent functions like is_directory()
• Mixing up is_file() and is_dir()
• Missing parentheses in function call

3. What will the following PHP code output if the file 'example.txt' exists and is 150 bytes in size?

$file = 'example.txt';
if (file_exists($file)) {
    echo filesize($file);
} else {
    echo 'File not found';
}

medium

A. 150

B. Error

C. 0

D. File not found

Solution

1. Step 1: Check file existence

   The code uses file_exists() to confirm 'example.txt' exists, which it does.

2. Step 2: Output file size

   Since the file exists, filesize($file) returns 150, which is printed.

3. Final Answer:

   150 -> Option A

4. Quick Check:

   File exists, output size = 150 [OK]

Hint: filesize() returns size only if file_exists() is true [OK]

Common Mistakes:

• Assuming filesize() works without checking existence
• Confusing filesize() output with file content
• Expecting 'File not found' when file exists

4. Identify the error in this PHP code snippet that checks if a file is writable:

$filename = 'data.txt';
if (is_writeable($filename)) {
    echo 'Writable';
} else {
    echo 'Not writable';
}

medium

A. Missing semicolon after echo statements

B. is_writeable() does not check permissions

C. $filename should be in double quotes

D. The function name is misspelled; should be is_writable()

Solution

1. Step 1: Check function spelling

   The correct function to check if a file is writable is is_writable(), not is_writeable().

2. Step 2: Verify other syntax

   Semicolons are present, variable quotes are valid, and is_writable() does check permissions.

3. Final Answer:

   The function name is misspelled; should be is_writable() -> Option D

4. Quick Check:

   Correct spelling = is_writable() [OK]

Hint: Remember: writable has one 'e' after writ [OK]

Common Mistakes:

• Misspelling is_writable() as is_writeable()
• Thinking variable needs double quotes
• Confusing permission check with existence

5. You want to list all files in a directory 'docs/' that exist, are readable, and have been modified in the last 24 hours. Which PHP code snippet correctly implements this?

hard

A. $files = scandir('docs/');
foreach ($files as $file) {
$path = 'docs/' . $file;
if (file_exists($path) && is_readable($path) && (time() - filemtime($path) < 86400)) {
echo $file . "\n";
}
}

B. $files = scandir('docs/');
foreach ($files as $file) {
if (is_dir($file)) continue;
if (is_writable($file)) {
echo $file . "\n";
}
}

C. $files = scandir('docs/');
foreach ($files as $file) {
if (file_exists($file) && filesize($file) > 0) {
echo $file . "\n";
}
}

D. $files = scandir('docs/');
foreach ($files as $file) {
if (is_file($file) && is_readable($file)) {
echo $file . "\n";
}
}

Solution

1. Step 1: Understand requirements

   We need to list files in 'docs/' that exist, are readable, and modified within last 24 hours (86400 seconds).

2. Step 2: Analyze each option

   $files = scandir('docs/'); foreach ($files as $file) { $path = 'docs/' . $file; if (file_exists($path) && is_readable($path) && (time() - filemtime($path) < 86400)) { echo $file . "\n"; } } correctly scans 'docs/', builds full path, checks file_exists(), is_readable(), and filemtime() against current time minus 86400 seconds. Other options miss one or more conditions or use wrong paths.

3. Final Answer:

   Option A code snippet -> Option A

4. Quick Check:

   Check existence + readability + recent mod = $files = scandir('docs/'); foreach ($files as $file) { $path = 'docs/' . $file; if (file_exists($path) && is_readable($path) && (time() - filemtime($path) < 86400)) { echo $file . "\n"; } } [OK]

Hint: Combine file_exists(), is_readable(), and filemtime() with full path [OK]

Common Mistakes:

• Not using full path when checking files
• Ignoring file modification time condition
• Checking is_writable() instead of is_readable()