PHP combine logical operators in IF clause

How do I successfully construct the following pseudocode IF clause in php ?

if $a is NOT equal to 'B'
OR
if $a is NOT both a string AND equal to 'C' or 'D'
continue

Just to clarify the second clause

• if $a is equal to C or D but not a string, it continues
• if $a is not equal to C or D even if it is a string, it continues
• only if $a is equal to C or D AND a string it does not continue

This are my attempts, but I am not negating correctly the second clause

if ( ($a != 'B') OR !(in_array($a, array("C", "D")) AND is_string($a) ) ) {      
  ...
}

and

if ( ($a != 'B') OR (!is_string($a) AND in_array($a, array("C", "D")) ) ) {      
  ...
}

Thank you