limit daily records

the solution was:

$sql="SELECT COUNT(*) AS 'Registros' FROM cita_previa WHERE 
fecha_cita='$fecha_cita'";
$res=mysql_query($sql,$link) or die ( mysql_error());
$nrows=mysql_num_rows($res);
list( $no_registros ) = mysql_fetch_array($res);
if( $no_registros >= 2 ) {

Use the count () function to know the number of lines in the list.

$sql="SELECT * FROM cita_previa WHERE fecha_cita='$fecha_cita'";

$res=mysql_query($sql,$link);
#$nrows=mysql_num_rows($res);
if(count($res)<=15){ # utiliza count()

on the other hand the mysql_ * functions are OBSOLETE using a PDO object Good luck!

EDITED

I leave you an example with a PDO object, a query function and the use of COUNT () outside the query.

PDO dbcore.php stackoverflow Warning ! may contain tabulation errors if you copy the following

<?php
Config::write('db.host', 'localhost');
Config::write('db.port', '3306');
Config::write('db.basename', 'puravida');
Config::write('db.user', 'root');
Config::write('db.password', '');

class Config {

    static $confArray; 

    public static function read($name) {
        return self::$confArray[$name];
    }

    public static function write($name, $value) {
        self::$confArray[$name] = $value;
    }
}

class Core {
    public $dbh; // manejador de conexion de la base de datos.
    private static $instance;

    private function __construct()  {

        $conexion = 'mysql:host=' . Config::read('db.host') . ';dbname=' . Config::read('db.basename') .';charset=utf8'. ';port=' . Config::read('db.port') .';connect_timeout=15';
        $user = Config::read('db.user');
        $password = Config::read('db.password');
        $this->dbh = new PDO($conexion, $user, $password,array(PDO::MYSQL_ATTR_INIT_COMMAND => "SET NAMES 'utf8'"));
        $this->dbh->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);// desactivar emulación de querys peparadors
        $this->dbh->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);// lanzar excepciones.
    }

    public static function getInstance() {
        if (!isset(self::$instance)) {
            $object = __CLASS__;
            self::$instance = new $object;
        }
        return self::$instance;
    }
}
<?php

FUNCTION agenda.php Warning ! may contain tabulation errors if you copy the following

    public static function Ver($idpaciente, $idpersonal)
    {
    if (!isset($idpaciente)) {
       $idpaciente=0;
    }
    if (!isset($idpersonal)) {
        $idpersonal=0;
    }

    // section 10-0-0-5-4286bd1e:14f806bdbcd:-8000:0000000000000C73 begin
    $qury='select DISTINCT
    agenda.idagenda as id,
    agenda.horainicio AS hora,
    agenda.horafinal AS hasta,
    salas.sala 
    FROM
    agenda
    INNER JOIN salasagenda ON salasagenda.idagenda = agenda.idagenda
    INNER JOIN salas ON salas.idsala = salasagenda.idsala
    INNER JOIN personal ON personal.idpersonal = agenda.idpersonal
    INNER JOIN pacientes ON pacientes.idpaciente = agenda.idpaciente
    INNER JOIN personas ON personas.ci = pacientes.ci
    INNER JOIN roles ON roles.idrol = pacientes.idrol
    WHERE
    (agenda.FechaFinal >='.date('Y-m-d').') AND (
    agenda.horainicio BETWEEN DATE_SUB(NOW(), INTERVAL "4" HOUR) and TIME("21:00:00")) and (agenda.idpaciente='.$idpaciente.' OR agenda.idpersonal='.$idpersonal.') ORDER BY agenda.horainicio';
    try {
    $bd=Core::getInstance();
    $qbdobj=$bd->dbh->prepare($qury);
    if ($qbdobj->execute()) {
        $qbdobj->setFetchMode(PDO::FETCH_ASSOC);
         return $qbdobj->fetchall();
         //print_r($qbdobj);
    }else{
        //print_r($qbdobj);
    }

USING COUNT () miagenda.php. Warning ! may contain tabulation errors if you copy the following

<?php
include $_SERVER['DOCUMENT_ROOT'] .'/Clases/class.Agenda.php';
$agenda=Clases_Agenda::Ver($idpaciente,$idpersonal);
?>
<div class="agenda">
<section>
    <h3>Hola <?php echo $nombre."! "; ?>para hoy <b><?php
    echo date('Y-m-d');  ?></b> tiene <b><?php echo count($agenda); ?></b> recordatorio(s);</h3><br>
    <?php 
    //print_r($idpaciente);
    //print_r($idpersonal);
    //$agenda= new Clases_Agenda();
    //print_r($agenda);
    if (!is_array($agenda)) {
        echo "No tienes nada para hoy.";
    }else{
         echo "<table border=1 width=100%><tr>";
         echo "<th>Desde hora</th>";
         echo "<th>Hasta hora</th>";
         echo "<th>Sala</th><th>acciones</th></tr>";
         for ($i=0; $i <count($agenda) ; $i++) {    
                echo"<tr><td>".$agenda[$i]['hora']."</td><td>".$agenda[$i]['hasta']."</td><td>".$agenda[$i]['sala']."</td>";
                //echo"<tr><td>".$agenda[$i]['hasta']."</td>";
                echo '<td><a href="agenda.php?url=modificar&id='.$agenda[$i]['id'].'">modificar</a></td>';
        }                       
             echo"</tr>";
         }
         echo"</table>";
         echo"<p></p>";

     ?>
     </section>
     </div>

Also if you notice that I am checking that $ agenda is an array.

if (!is_array($agenda))

luck!

Friend, my answer, similar to the previous ones, I also add that you can occupy the functions of mysqli instead of those of mysql, I leave this link: When you stopped using mysql_comando commands and what version of php ?

$sql="SELECT * FROM cita_previa WHERE fecha_cita='$fecha_cita'";

$res=mysql_query($sql,$link);
#$nrows=mysql_num_rows($res);
if(count($res)<15){ # utiliza count()

I saw that in all the examples (answers), even in your question it says $res <= 15 in strict rigor if you want the maximum to be 15 should be so $res < 15 or in the case of this example count($res) < 15 If not, when you reach 15 you will enter one more record.

I hope it serves you.

This is to answer the question, and to give as an example according to the conversation of this answer.

Assuming you have a table cita_previa and it has 15 records where in the field fecha_cita , of all records has the value of 2010-10-10 . Then there is a daily limit of records, where the limit is 15 records, that is, in this example, no more records should be allowed for the 2010-10-10 date.

Then in the PHP page that enters records we will have.

<?php 

// donde $_POST['fecha'], viene de un formulario enviado por POST y el valor de este es '2010-10-10'
$fecha_cita = isset($_POST['fecha']) ? $_POST['fecha']: '';

// Comprobamos que ya no hayan mas de 15 registros con esa fecha.
$rs = mysql_query("SELECT * FROM tabla WHERE fecha = '$fecha_cita'", $link)
// Basicamente la consulta dice 'SELECT * FROM tabla WHERE fecha = '2010-10-10''
// comparamos la cantidad con el numero que le pusimos de limite
if(mysql_num_rows($rs) < 15){ 
// si habiamos dicho que teniamos 15 registros con esa fecha, estara preguntando si 15 < 15, como no lo es, irá al else.
// si es menor, ingresará uno nuevo
} else {
// si es 15 o mayor a 15 no ingresará un nuevo registro.
}

I hope it can work.

I do not advise you to keep using MySQL* , they are insecure against SQL injection .

  

WARNING: MySQL * was declared obsolete in PHP 5.5.0 and removed in PHP 7.0.0. Instead you should use the extensions MySQLi or PDO_MySQL .

Let's look at an example MySQLi:

//Fecha obtenido desde tu formulario.
$fecha_cita = $_REQUEST['fecha_cita'];

//Sentencia.
$sql = mysqli_query($link, "SELECT cedula FROM cita_previa WHERE fecha_cita='$fecha_cita'";
//Comprobamos existencia de registros en Base de datos y mientras sea inferior a total de (3) registros, insertamos un registro nuevo.
if(mysqli_num_rows($sql) < 3) {
    //Insertamos registro nuevo a la Base de datos.
} else {
   echo 'Tu registro no se inserto, la Base de datos ya cumple con los regisrtos permitidos.';
}

I advise you to use sentences prepare () or PDO , to achieve greater security in your web application.

Example mysqli_prepare ():

//Fecha obtenido desde tu formulario.
$fecha_cita = $_REQUEST['fecha_cita'] ?: '';

//Sentencia (SELECT).
$stmt = $link->prepare("SELECT cedula FROM cita_previa WHERE fecha_cita = ?");
$stmt->bind_param("s",$fecha_cita);//Ligamos parametros marcadores, es decir los valores ?.
$stmt->execute();//Ejecutamos sentencia.
//Comprobamos registros.
$stmt->store_result();
if($stmt->num_rows < 3) {

   $stmt->close();//Cerramos sentencia.

   //Sentencia (INSERT).
   $stmt = $link->prepare("INSERT INTO cita_previa (cedula,idtipodecita,fecha_cita,estado,telefono) VALUES(?,?,?,?,?)";
   $stmt->bind_param("iisss",$cedula,$idtipodecita,$fecha_cita,$estado,$telefono); //Atención, iisss,  son los tipos para el correspondiente enlazado de variable, deben de coincidir.

   //i     la variable correspondiente es de tipo entero
   //d     la variable correspondiente es de tipo double
   //s     la variable correspondiente es de tipo string
   //b     la variable correspondiente es un blob y se envía en paquetes

   $stmt->execute(); //Ejecutamos sentencia
   $stmt->close(); //Cerramos sentencia.
   echo 'Tu registro se inserto correctamente a la Base de Datos.';

} else {
    $stmt->close();//Cerramos sentencia.
   echo 'Tu registro no se inserto, la Base de datos ya cumple con los regisrtos permitidos.';
}

I think you should make a query using the COUNT function of mysql to tell you the records that meet the following criteria WHERE fecha_cita = 'Fecha_yday';

if the number of records is equal to or greater than 15 you throw the message that there are already more than 15 records and you are ready