<?php

function domingosNoMes( $mes, $ano ) {
	$t = gmmktime( 0, 0, 0, $mes, 1, $ano );
	$ult = date( 't', $t );
	$sem = date( 'w', $t );
	return floor( ( $ult - 1 ) / 7 ) + ( $sem == 0 || $sem > ( 35 - $ult ) );
}

// teste
for( $ano = 2015; $ano <= 2017; ++$ano ) {
	for( $mes = 1; $mes <= 12; ++$mes ) {
		echo $mes.'/'.$ano.' - '.domingosNoMes( $mes, $ano ).PHP_EOL;
	}
}