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<?php $tests = [ 'аяruski', 'string', '54345', 'é', 'аяruski foo', 'string foo', '54345 foo', 'é foo', ]; var_export( preg_replace_callback( '/^(.)(.*)/u', fn($m) => mb_strtoupper($m[1]) . mb_strtolower($m[2]), $tests ) );
Output for 8.1.0 - 8.1.33, 8.2.0 - 8.2.29, 8.3.0 - 8.3.25, 8.4.1 - 8.4.12
array ( 0 => 'Аяruski', 1 => 'String', 2 => '54345', 3 => 'É', 4 => 'Аяruski foo', 5 => 'String foo', 6 => '54345 foo', 7 => 'É foo', )
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