Answer
Both include a file only once. include_once gives a warning on failure, require_once causes a fatal error.
Quick Answer
include_once and require_once track which files have been loaded and skip re-loading them. This prevents errors from redeclaring classes or functions. require_once causes a fatal error if the file is not found; include_once shows only a warning. Use require_once for class files and libraries where double-loading would cause redeclaration errors.
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SugharaIQ Editorial Team
Verified Answer
This answer has been peer-reviewed by industry experts holding senior engineering roles to ensure technical accuracy and relevance for modern interview standards.
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Source: SugharaIQ