The while ($line = fgets($file)) pattern is not a typo but a deliberate idiom where assignment returns the assigned value, which is evaluated for truthiness in the loop condition. 2. The loop continues as long as fgets() returns a truthy value (i.e., a valid line, even if it's an empty or "0" line), and stops only when fgets() returns false at end-of-file. 3. This works because PHP treats most strings as truthy, reserving falsiness for false, "", "0", null, etc., making false the sole reliable terminator. 4. Avoid wrapping the assignment in functions like trim() or intval() directly, as they may convert valid input into a falsy value (e.g., "0" becomes falsy), causing data loss. 5. Best practice is to perform such transformations inside the loop body after the assignment to preserve correctness. 6. While idiomatic and efficient for file reading, database fetching, and regex matching, consider using an explicit while (true) with a break condition if code clarity is a priority for team maintainability.

You’ve probably seen this pattern in PHP:

while ($line = fgets($file)) {
    echo $line;
}

At first glance, it might look like a typo—shouldn’t that be == for comparison? But no, this is a deliberate and common idiom. Let’s break down what’s really happening and why it works.

How Assignment Works Inside while

In PHP, an assignment like $line = fgets($file) doesn’t just assign a value—it also returns the assigned value. So:

$line = fgets($file)

doesn't only store the line in $line, it also evaluates to the value of $line (i.e., the line read from the file, or false when there's nothing left to read).

The while loop checks the truthiness of that returned value. So the loop continues as long as fgets() returns a string (which is truthy), and stops when it returns false (end of file).

Why It Works: Truthiness and Falsiness

PHP uses loose boolean evaluation in conditionals. Here's what matters:

• A successfully read line (even "0\n" or a blank line like "\n") is considered truthy.
• Only when fgets() hits the end of the file does it return false — which is falsy, so the loop exits.

This means even empty lines are processed, which is usually what you want.

Compare this to a flawed version:

while ($line = trim(fgets($file))) {
    // Problem: if line is "0", trim() returns "0", assignment returns "0"
    // In a condition, "0" is falsy → line gets skipped!
}

So be careful: wrapping the assignment in functions like trim() or intval() can cause valid data to be treated as falsy.

Common Use Cases

This idiom appears in several I/O and iteration contexts:

• Reading files line by line:

  while ($line = fgets($handle)) { ... }

• Fetching database rows:

  while ($row = mysqli_fetch_assoc($result)) { ... }

• Iterating over regex matches:

  while (preg_match($pattern, $text, $matches)) { ... }

All rely on the same principle: assignment returns a value, and the loop continues while that value is truthy.

Best Practices and Pitfalls

Here are a few things to keep in mind:

• ? It’s idiomatic and efficient — no need to call feof() or pre-fetch outside the loop.
• ?? Don’t modify the assignment result — avoid trim(), strtolower(), etc., directly in the assignment if the result could be falsy.
• ? If you need to process the value, do it inside the loop:

  while ($line = fgets($file)) {
      $line = trim($line); // Safe here
      // process $line
  }

• ? Avoid confusion — if readability is a concern (e.g., for junior developers), split it:

  while (true) {
      $line = fgets($file);
      if (!$line) break;
      echo $line;
  }

  ---

  Basically, the while ($line = ...) idiom is safe, fast, and widely used in PHP for stream and result handling. It’s not a quirk—it’s a feature of how PHP evaluates assignments. Just remember: it works because assignment returns a value, and the loop runs while that value is truthy.

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