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In PHP development, we often need to compare strings, especially when dealing with passwords, hashes, signatures, and other security-sensitive data. Developers may be tempted to use “==” for simple string comparisons. However, when it comes to comparing hash strings, PHP provides a dedicated function hash_equals(). Why does PHP discourage using “==” instead of hash_equals(), and recommend the latter instead? Let’s take a closer look at the reasons.
In PHP, == is a loose comparison operator, meaning it automatically converts operand types before comparison. This behavior can lead to unexpected results when dealing with numbers and strings. For example:
<span><span><span class="hljs-variable">$hash1</span></span><span> = </span><span><span class="hljs-string">"abc"</span></span><span>;
</span><span><span class="hljs-variable">$hash2</span></span><span> = </span><span><span class="hljs-string">"a"</span></span><span> . </span><span><span class="hljs-string">"bc"</span></span><span>;
<p></span>var_dump($hash1 == $hash2); // Output: bool(true)<br>
</span>In the above example, $hash1 and $hash2 don’t look identical at first glance. However, since PHP performs type juggling during loose comparisons, it evaluates these two strings as equal.
More importantly, PHP compares strings character by character until it finds a difference. But this type of loose comparison is not suitable for hash values, because hashes represent binary data and demand strict accuracy for each character. Even the slightest difference should make hashes unequal. Therefore, == is not appropriate for hash comparisons.
Unlike ==, hash_equals() is specifically designed for comparing hash values. Its behavior is strict: it does not perform type conversion and is unaffected by PHP’s loose comparison rules. hash_equals() checks each byte of the two strings to ensure they are exactly the same. If there’s any difference, it immediately returns false, without being influenced by type or other factors.
Additionally, hash_equals() mitigates timing attacks. A timing attack is a method where an attacker deduces sensitive data by analyzing differences in execution time. If you use == for hash comparison, PHP’s evaluation process might leak information that attackers can exploit. In contrast, hash_equals() completes the comparison of all characters before returning a result, preventing this vulnerability.
Because == is a loose comparison, it may behave unpredictably when handling hash values. For instance, if you compare two hashes of different lengths using “==”, PHP might pad or evaluate them in unexpected ways, leading to false matches. hash_equals() avoids this issue by strictly comparing each byte to guarantee equality.
One of the risks of using == for hashes is that it may inadvertently reveal information about string lengths. If PHP processes the strings differently or applies implicit conversions, attackers could exploit timing differences to infer whether certain hashes match.
hash_equals() provides constant-time comparison, meaning the execution time is the same regardless of the input values. This is crucial for defending against timing attacks.
While hash_equals() may be slightly slower than ==, its security benefits far outweigh the performance cost. Especially when handling passwords or sensitive data, security should always take precedence over speed. Even if == runs faster in some cases, hash_equals() is still the recommended choice for hash comparisons.
Moreover, PHP has optimized the implementation of hash_equals(), ensuring its performance is efficient enough in most scenarios.
In PHP, although “==” is a common comparison operator, it is not suitable for comparing hash values, especially in security-critical situations. hash_equals() is purpose-built for hash comparison, offering stricter, safer validation and avoiding the pitfalls of loose comparisons. To ensure code security—particularly when dealing with passwords, API keys, or digital signatures—developers should always use hash_equals() instead of == when comparing hash values.
