PHP - First and last character check

I am trying a check in PHP. What I want is to check:

• The first character in a string must be a "["
• The last character in a string must be a "]"

I've written this code (last if statement is what I actually want) to check this but am not getting the result I wanted:

$str = "dfgdfg]";

if($str[0] != '[') {
    echo "FIRST NOT EQUAL TO";
}

if(substr($str,-1) != ']') {
    echo "SECOND NOT EQUAL TO";
}


if( ($str[0] != '[') && (substr($str,-1) != ']') ) {
  echo "COND MET!";
}

If I run this code I thought that the last if statement would execute however, only the first if statement executes. What is wrong with the logic in the last if statement?