A namespaced call falls back to the global function, at a cost

Inside a namespace, an unqualified call to count() is not a call to count(). PHP looks for AppImportcount() first, fails to find it, and only then falls back to the global function. The fallback is what makes namespaced code work at all without importing half the standard library, and it happens on every call.

namespace AppImport;

// two lookups at runtime: AppImportcount, then count
$n = count($rows);

// one, resolved when the file is compiled
$n = count($rows);

// constants fall back the same way
echo PHP_EOL;      // PHP_EOL, after a failed look for AppImportPHP_EOL

The performance argument is real and small: a failed hash lookup followed by a successful one, which is measurable inside a loop running a few million times and invisible everywhere else. The correctness argument is the stronger one. Because the fallback is resolved at runtime, defining AppImportcount() tomorrow silently changes the meaning of every unqualified count() already written in that namespace — no error, no warning, different behaviour. A leading backslash pins the call at compile time and costs nothing. The habit worth forming is the backslash on functions and constants in namespaced files; classes are unaffected, since they never fall back to global scope in the first place.