1. Block comments in PHP are processed by the lexer, which scans all content between / and / despite not generating opcodes. 2. This scanning causes a small parsing overhead, especially with large comments, impacting parse time without opcache. 3. With opcache enabled, comments are stripped after compilation, resulting in zero runtime performance cost. 4. Large comments increase file size, affecting disk I/O, memory usage, and tooling performance. 5. Avoid nesting block comments as PHP does not support it, leading to parsing errors. 6. Best practices include using PHPDoc for documentation, deleting dead code instead of commenting it, and keeping comments concise to ensure maintainability and efficiency. Therefore, while block comments are safe for normal use, they should not be overused or abused.

Block comments in PHP—like /* ... */—are commonly used to disable code or add multi-line documentation. While they seem harmless, their interaction with PHP’s parser and potential impact on performance is worth understanding, especially in large-scale applications or performance-critical contexts.

How PHP's Parser Handles Block Comments

PHP’s parser processes source code in a lexical analysis phase before compilation. During this phase, comments—both block (/* */) and line comments (//, #)—are recognized as non-code tokens and are typically discarded before the actual parsing of executable statements.

However, block comments are not ignored during scanning. The lexer must still:

• Identify the opening /*
• Scan every character until it finds the closing */
• Validate proper nesting (though PHP does not support nested block comments)

This means the parser does work on block comments—it just doesn’t generate opcodes from them. For very large block comments (e.g., thousands of lines), this scanning adds a small but measurable overhead during script compilation.

Example: A 10,000-line block comment in a file forces the lexer to process 10,000 lines of text it will ultimately discard.

Performance Implications

The performance impact of block comments is generally negligible in production environments if opcache is enabled, but here’s a breakdown of when it matters:

• Without OpCache (Development/CLI scripts)
  Every request triggers a full re-parse of PHP files. Large block comments increase parse time slightly because the lexer must scan through all comment content.

• With OpCache (Production)
  Once a script is compiled and cached, comments are already stripped from the opcode representation. Subsequent requests use the cached opcodes, so comments have zero runtime performance impact.

• File Size & I/O Overhead
  Even if parsing is fast, bloated files with huge comment blocks increase disk I/O and memory usage when loading files. This affects:

  • Time to read the file from disk
  • Memory footprint during parsing
  • Version control diffs and tooling performance

Best Practices to Minimize Impact

While block comments are safe for small uses, avoid these patterns:

• ? Use for short explanations or temporary code disable
• ? Don’t comment out large blocks of legacy code—just delete them (use version control instead)
• ? Avoid embedding documentation or SQL dumps in /* */ blocks
• ? Use PHPDoc annotations (/** */) for documentation—tools expect them and they’re structured

Also note: PHP does not support nested block comments. This causes parsing errors:

/*
  /* This breaks */
*/

Bottom Line

Block comments have minimal performance cost in real-world applications thanks to opcache, but they’re not "free" during parsing. The main risks are:

• Increased parse time for huge comments (without opcache)
• Poor code maintainability
• Larger file sizes affecting I/O

So while the parser handles them efficiently, the best practice is simple: keep comments lean, meaningful, and remove dead code entirely.

Basically, don’t fear /* */ for normal use—but don’t abuse them either.

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