尝试

$txt = preg_replace("/^(.*)(\\1+)$/", "", $txt);

字符串的开始，然后匹配该字符串的至少一个重复，然后匹配字符串的末尾。

This searches for a sequence at the start of the string, then matches at least one repetition of that string, then matches the end of the string.

例如

$txt = preg_replace("/^(.*)(\\1+)$/", "", "cupcupcup");  //=> ""
$txt = preg_replace("/^(.*)(\\1+)$/", "", "cucupcup");   //=> "cucupcup"