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<?php $name = 'FOO'; $value = false; putenv("{$name}={$value}"); var_dump(getenv($name)); $value = true; putenv("{$name}={$value}"); var_dump(getenv($name));
Output for 5.6.0 - 5.6.40, 7.0.0 - 7.0.33, 7.1.0 - 7.1.33, 7.2.0 - 7.2.33, 7.3.0 - 7.3.33, 7.4.0 - 7.4.33, 8.0.0 - 8.0.30, 8.1.0 - 8.1.28, 8.2.0 - 8.2.19, 8.3.0 - 8.3.7
string(0) "" string(1) "1"

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