PHP file_get_contents然后替换文件中的字符串

I am using this

file_put_contents('/usr/local/file.cfg', str_replace(  "mypassword:=$one", "mypassword:=$two",  file_get_contents('/usr/local/file.cfg')    ));

to change mypassword:= value from value $one to value $two

BUT , inside file /usr/local/file.cfg I have also arx_mypassword=arx_value which is changed too using the command above, but I do not want change it .

Can I change mypassword value without changing arx_mypassword value too ?

Thank you