Why Does `strpos()` Return 0, and How Can I Reliably Check for Substring Existence?

Understanding Return Value Quirks of strpos()

When using strpos() to locate a substring within a given string, it's crucial to consider the return value implications. Failure to do so can lead to unexpected outcomes.

Issue:

In an attempt to identify the starting position of two substrings and ensure the first is located before the second, a programmer encounters an unexpected behavior. Code checks for the presence of each substring in the target string using the loose equality comparison operator (<>). If the return value of strpos() is false, it implies that the substring was not found. However, when the substring starts at position zero, it appears that strpos() returns 0, which is incorrectly interpreted as false.

Resolution:

The PHP manual explicitly states that strpos() can return Boolean FALSE or non-Boolean values (e.g., 0) that evaluate to FALSE. To avoid this confusion, PHP recommends using the === operator (strict equality) for testing the return value of strpos().

In the example provided, the programmer should replace the loose equality operator (<>) with the strict equality operator (!==) in the strpos() checks. This will ensure that the comparison operates as expected, correctly identifying the presence or absence of the substrings in the target string.

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