Guesstimate: if someone cares about performance, they should have already installed an opcode cache, which probably wouldn't have to re-parse constant strings.

Guesstimate: if someone cares about performance, they should have already installed an opcode cache, which probably wouldn't have to re-parse constant strings. But then thinking: you'd expect constant double quoted strings to also get this treatment, so single quoting still gets you nothing. All the above is void without benchmarks.